important notes
None of this information is ripped. All of it is artisanal hand-gathered playground information. Any of it might be wildly incorrect or incomplete, especially at the beginning of a month when new critters are available. That's part of the fun!
The tips deliberately do not contain major spoilers, advice on intense grinding, or anything you could possibly construe as an exploit — e.g., manipulating spawns or resetting a tool's durability. They're intended as tips for newer players, not a gamefaq for turning the game into a Bell printer.
Apologies to players in the southern hemisphere, but I can't add a southern hemisphere mode — I'm up north and entering the critters by hand, so I won't see the critters in your game for six months!
The fish and bug rarities —
— are entirely subjective and based on my own experience.
Corrections welcome, but I'm hesitant to include anything I haven't seen with my own eyes.
Last updated on Apr 5, 2020.
credits
Most of the work done by Eevee (@eevee).
The Animal Crossing font is FOT-Seurat Pro from Fontworks, acquired from the Internet Archive and stripped of kanji for filesize reasons using FontForge.
Animal Crossing item icons were obtained from The Spriters Resource (warning: this is a full sheet, so possible spoilers).
basic gameplay
As far as the game is concerned, the day changes at 5 AM. If the game says it'll happen "tomorrow", it happens at 5 AM.
Once you've run out of new things to do, you'll probably settle into a routine of "making the rounds" — checking everything on your island that changes every day.
- 4 buried fossils appear randomly.
- One glowing spot appears randomly, containing
1000. - Fruit trees bear fruit three days after they were last picked.
- Cedar and hardwood trees are populated with 5 wasp nests, 2 items of furniture, and 12 or so coins worth 100.
- All trees "refill" with wood.
- Rocks "refill" with items.
- Watered flowers reproduce.
- Inventory at shops changes.
- The available fence recipes and special catalog items at the Nook Stop change.
- You can get a new Nook Stop bonus.
Other (non-seasonal) events that happen when the day changes, but not necessarily every day, include:
- Young trees and flowers grow up by one stage.
- Construction projects finish, or sometimes start.
- Villagers move in or out.
- Particular NPCs may arrive or leave.
- Mail from NPCs and catalog orders are delivered.
Time is local to the Switch being played on, so even if (e.g.) your shop is closed, a friend's might still be open.
The weather can only change on the hour. If it's raining, it'll keep raining until (at least) the next hour.
Island tours have their own weather, which I believe is shared between all of them, and which also only changes on the hour.
You can rearrange your pockets by using A plus a direction. It's a little fiddly: if you press A and then a direction immediately, you'll pick an item up; if you hold A for a moment, you'll pick it up in place; if you press A and then a direction too slowly, the cursor simply moves as normal.
Press Y to fill holes without needing a shovel.
Be careful of using Y on holes near flowers; the game will prefer picking the flowers to filling in the hole! In general, the game prioritizes picking up dropped items, then picking flowers, then taking furniture or filling holes.
You can jump over holes, hop between diagonally-placed holes, or squeeze through a diagonal gap by mashing yourself in the appropriate direction.
Try pole-vaulting over a river and directly into a hole.
Pressing A will generally use the tool you're holding, or talk to an NPC. There are a few exceptions where A will do other contextual actions (like shaking a tree), most notably the net and fishing rod. Press down on the d-pad to put your tool away.
Most objects you place go in front of you, but flowers and weeds and custom patterns go underneath you.
Walk up to furniture, hold A to grab it, and move the stick forwards or backwards to push or pull it by half a tile at a time or sideways to rotate it.
You can only order 5 catalog items from the Nook Stop per day, including the special items. You can cheat this limit, very slightly, by mailing up to 2 items per day to friends. Mailed items show up instantly, rather than overnight.
If your island announcements mention a meteor shower, you can wish on shooting stars all night, starting at 7 PM! With your hands empty, tap up on the right stick until you're looking at the sky, then press A when you see a shooting star to wish on it. Shooting stars tend to come in clusters, so wait a few seconds and you'll probably see more. Star fragments will wash up on your beach tomorrow (even if you wished on another player's island); in my experience, you get one for every four wishes you make.
Make sure to empty your hands before looking up, especially if you're rushing to catch a star you just heard. If you look up first and then realize you're holding a tool, putting it away will reset the camera, and you won't have enough time left.
You need star fragments to make wands, which let you do a magical girl transformation and change your entire outfit at once. The actual clothes in your inventory don't change, and you can't change your clothes directly while transformed.
Clothing items are stored in wand outfits they're assigned to, so you can't use the same clothes in multiple outfits (unless you have duplicates). On the other hand, making 8 outfits is a good way to clear out some storage space if you're a hoarder like me.
Chairs can be sat upon by moving into them. Beds can be laid upon the same way.
Stereos, record players, and the like can play albums. Sadly, it seems that every music player you place outside is set to the same song.
Wardrobes allow you to change clothes directly from storage, even when placed outside. Refrigerators and chest freezers count as wardrobes; I don't know why.
Sitting on a toilet will clear out your fruit meter. Toilets you can't sit on — including the Japanese toilet, urinal, and portable toilet — don't work this way.
Trash cans allow you to permanently destroy items.
Some other furniture can be interacted with or turned on/off by pressing A.
bugs, and some non-bugs
A few bugs are especially skittish, but you can sneak up on them by tilting the stick only slightly — or just hold A with the net out to tiptoe.
The 
tarantula is very aggressive.
If you start to approach it, it'll rear up and hiss at you; any further and it'll chase you and knock you out!
To catch it, simply stop moving until it lowers its legs, inch forwards until it rears up again, and repeat until you're in range with the net.
Bugs come in a huge variety and can appear in all kinds of unexpected places, so keep an eye out, and make sure your net is quickly accessible.
Some bugs only appear when shaking trees.
Some bugs only appear on stumps (left behind after cutting a tree down with an axe).
Some bugs only appear near flowers.
Some bugs only appear near hybrid flowers.
Some bugs only appear near particular colors of hybrid flowers.
Some bugs only appear on rocks, or when you hit rocks.
Some bugs only appear at certain times or during certain weather.
Some bugs only appear when certain items are left dropped on the ground.
Some bugs only appear on villagers!
fish, mostly
You can get a sense of what fish have what size of silhouette, and then what silhouettes are most valuable.
To my eyes, at least, there are five sizes: XS, S, M, L, XL.
Fish change every month, but you can develop a sense of which silhouettes aren't worth pursuing in which bodies of water.
For example (at least in March when the game came out), an XS fish in the sea is always a 
sea butterfly which goes for 
1000, so they're always going after. But an L fish is quite often a 
sea bass, which sells for a less exciting 400. You might get a 
football fish worth 2500, but if you're in a hurry or on a mystery tour with limited pocket space, you might opt to skip L fish altogether.
I strongly suspect that it's better to chase away fish you don't want than to leave them along, since that frees up a spawn slot. I don't know the exact spawn mechanics, so maybe there aren't spawn slots, but however it works, getting rid of fish must surely cause new fish to appear sooner.
Fish seem to appear more frequently when it's raining. On mystery tours, either fish appear more frequently, or they appear at the same rate but in a smaller area; it's hard to tell, but either way, they're good for fishing. Some mystery tours also have especially active fish spawns; I've found Carp World, with a large pond that had 3 or 4 fish in it constantly.
Some fish appear only at the pier or river mouth (the part of the river passing through the beach), so check those out whenever you run by, and always catch whatever's there.
Some fish only appear in rivers on cliffs, i.e. that you need to use a ladder to reach. Some of them are exceptionally rare, and cliff rivers tend to be small so they don't repopulate very quickly, so it might be worth making a few stacks of bait and spending it all there — at different times of day! — a couple times per month.
miscellaneous money
You earn interest on your bank balance every so often. The bank will send you a letter when this happens. (I received some on April 1, but have also heard it happens weekly?) Alas, the interest rate is 0.5%, which is practically nothing, so this isn't really worth it.
You don't have to grind to make money! You can get a good bit of cash every day just by spending a few minutes making the rounds.
5 wasps sell for 
12500. See the tips on trees.
The money rock produces 15900. See the tips on rocks.
The money tree you planted four days ago produces 20000 net profit. Be sure to plant a new one. See the tips on trees.
Foreign fruit adds up really fast. Even 10 foreign fruit trees (a mere 3 stacks of fruit) will make you 15000.
That's 63400, every day, just from shaking trees and hitting rocks.
Try to get a sense of how much fish and bugs sell for. You don't need to memorize exact amounts, but you should have an idea of whether a given critter is worth catching. The night drop slot outside the shop is good for this, since you can instantly see how much an item sells for. It's 20% lower than the normal sale price, so not exactly correct, but you can still see the difference between selling for hundreds versus thousands.
(You should also try not to sell especially lucrative catches through the drop box; save them for tomorrow! 20% of tens of thousands is still thousands!)
For example, ignore:
honeybees,
ladybugs,
normal stinkbugs,
common/yellow/tiger butterflies,
moths, and
citrus beetles.
All of them only sell for a few hundred Bells at best and aren't really worth the pocket space.
But if you see an 
orchid mantis, you should drop whatever you're doing and go for the 2400.
Plant a lot of flowers!
They attract
orchid mantises (2400),
man-faced stinkbugs (1000),
paper kite butterflies(1000),
and all kinds of new flutterers as of April.
Plant or breed blue, purple, or black flowers if you can, and put them somewhere you walk by frequently (shoreline, shop, house, etc.) — during the day, they attract a constant stream of 
peacock butterflies, which sell for 2500!
It's free money.
I cannot stress enough to always have your net out when shaking trees, so you can catch any wasps. See the tips on trees.
Always go for XL ocean fish; they're rare, and every single one of them sells for thousands.
Fish a lot in the rain; you'll probably find a couple 
coelacanths, each worth a whopping 15000.
If you want to make a LOT of cash, play the stalk market.
Every Sunday morning (5 AM to noon), Daisy Mae will sell you turnips for 90–110 per bunch of 10.
You can then try to sell them at the Nooklings' shop for a profit, before they rot next Sunday morning.
The sell price changes at the start of each day (except Sunday) and again at noon.
Turnip prices follow one of a handful of known patterns, so it's possible to figure out which pattern you're seeing and predict the future to some extent.
But one of the patterns is a slow decline that will never turn you a profit, so there's a much simpler way.
Make friends with a few folks who also play the game!
Among all of you, there's a pretty good chance at least one of you will see a price of 300 or more sometime during the week, and you've tripled your investment.
Most usual investment advice applies. Don't invest anything you're not willing to lose. Get more conservative as you approach the end of the week; selling for a small loss is better than losing them entirely.
You cannot put turnips in storage, so you have to drop them somewhere — ideally where visiting players can't steal them. Other players can't pick up dropped items in your house, so leaving turnips there is safe. You can also safely store them outside if you, e.g., build a fence around them; visitors can't move your furniture. Of course, you could just only allow visitors you trust not to steal your stuff.
You can eat turnips, since they're classified as a fruit, sort of.
Each one is actually a bunch of 10, so eating "one" will immediately fill your fruit meter to full.
But that's 1000 worth of turnips, so, probably try not to do that.
flower power
Running through flowers only destroys the blossom; it'll regrow in two days.
Pressing Y only picks the blossom; you need to use the shovel to pick up and move flowers.
Unwatered flowers don't wilt as in previous games! Watering only encourages flowers to reproduce.
Don't bother watering flowers until the end of the day, or at least until you're done playing. It might rain later, which waters everything automatically.
Rain also moistens the soil, so planting fully-grown flowers (e.g., ones you had in storage or swiped from a mystery tour) even after it's rained will automatically water them.
You can crossbreed colors of flowers (of the same species) to get new colors. Plant them near each other, water them with the watering can (or hope it rains), and pray. Your watering can covers a 2×3 area centeredon you, but alas, that means maybe three flowers at a time, since you'll want extra space for new flowers.
"Near each other" is pretty flexible. I'm not 100% confident, but it seems that two flowers can breed as long as the new flower and the two parents all end up touching somehow. I've seen both of these patterns happen, where the pink flower is the new one:
Common approaches for encouraging breeding are rows of solid color or a checkerboard pattern. The most important thing is to leave empty space for new flowers to grow. However, yellow and white generally only makes yellow, so you may want to keep them separated. I've been doing this:
Colors vary per flower, but the obvious combinations work: red + yellow can make orange; red + white can make pink. Additionally, white + white can make blue, red + red can make purple or black, and a few other colors exist. I have no idea how windflowers work; unlike other flowers, they have orange as a "basic" color!
Your island has a native flower, which covered your cliffs when you started the game. It also has a second-place flower, which appears on mystery tours fairly frequently, and perhaps one or two runners-up. The seeds available in your shop will generally be from this set of preferred flowers.
If you want more diversity of flower types, visit friends' islands and buy flower seeds from their shops!
You can also steal flowers from mystery tours, if you're lucky enough to find an island with flowers you haven't seen before. If you're really lucky, you might even find an island with hybrid colors.
tree hugging
There are four kinds of trees:
fruit,
hardwood,
cedar,
palm.
Plus 
bamboo, I guess.
Fruit trees look identical to hardwood trees when not bearing fruit.
(In April, hardwood trees sprout cherry blossoms and fruit trees do not.)
Any given tree will drop up to 3 wood or bamboo per day when hit with an axe. Fruit, hardwood, and cedar trees will produce a seemingly endless supply of tree branches when shaken, though they may or may not drop a branch on any given shake.
Every day, the game hides 2 items of furniture, 5 wasp nests, and 12+ coins worth 100 among your hardwood and cedar trees.
The first time you shake a tree (press A with empty hands, or holding a rod/net) or hit it with an axe that day, if one of those goodies is in the tree, it'll fall out.
Fruit trees will never drop goodies, even if they have no fruit today. It's a good idea to distinguish between fruit and hardwood trees, so you know which ones to shake. Some folks label the ground in front of fruit trees with a custom pattern. I went a more aggressive route and destroyed all fruit trees on my ground level, so the ground is all fruit, and the cliffs are all cedar and hardwood.
Wasps can't spawn (again) from a tree after you've shaken it, so you should do a round of shaking all your trees before you use an axe on any of them.
Have the net out and stand diagonally in front while shaking; if a wasp nest drops, your surprise reaction automatically turns you to face it, so by mashing A you can catch the wasps before they even have a chance to move.
Each one sells for 2500, so that's 12500 per day in wasps alone.
To grow a new tree, dig a hole and plant a fruit or sapling in it. Trees need empty space around them in order to grow (in a + shape), or they'll remain as saplings. Use the shovel on a sapling to dig it up (you don't need fruit) and you can replant it elsewhere.
I'm fuzzy on the exact rules for tree spacing; it used to be that a tree's + shape couldn't overlap another's, so two adjacent trees had to have two spaces between them. But I've heard that trees can now be only one space apart, and you can definitely replant them that close, so maybe the rules have been relaxed. I don't recommend cramming trees too close together, though, or you'll have a hard time seeing what's dropped from them.
Every day, a random tile on your island will have a glowing spot that you can dig up to get 1000.
You can rebury money in that glowing hole to grow a money tree that will yield three times what you buried!
(In your pockets, move the cursor to your wallet and press A to grab some cash as an item.)
The money tree sprouts as a sapling instantly and grows at normal speed. Once you've harvested it once, it becomes a normal hardwood tree. If you'd like to avoid having a constant supply of trees growing in random places, you can dig it up right away (without fruit) and make a little rotating four-tree money forest.
I've heard conflicting reports of how much money a tree can grow, but it seems safe to bury a maximum of 10000, for a profit of 20000 four days later.
Your native fruit sells for a paltry 100.
Foreign fruit sells for 500 (except coconuts, which are 250), so make friends and plant a bunch of their fruit!
Or try mystery tours until you find an island with different fruit.
(Remember, you can eat fruit and use the shovel to dig up entire trees!
Take them home, replant them, and you don't need to wait for them to grow from seedlings.)
You can also sell your native fruit for 500 if you visit a friend's island with a different native fruit.
Beware of surrounding trees with flowers, weeds, or anything else! More than one object cannot coexist in the same tile, so if an item (even fruit) drops from a tree and can't find an empty tile to land in, it'll vanish, and you lose it.
rock stardom
You can get up to 8 items from a rock per day, but once you start hitting them, they'll only produce items for a short time (like Mario multi-coin blocks). To guarantee getting all 8, you have to block yourself in so the recoil doesn't push you away. (Also, be sure to pick up anything surrounding the rock; sometimes a lone stone will appear next to it.)
The easiest way to do this is by digging two holes in a corner (please use your imagination here):
Mash A, and you should get 8 items, the last one dropping right where you're standing. If you use holes, you can even jump over them and back to get a perfect 45° angle. For a more permanent solution, consider putting a few fence pieces, some furniture, a tree, etc.
You can also brace yourself against a single object and hit the rock from the side, but that's riskier — if your angle is even slightly off, you'll slide out of the way.
Every day, one rock on your island is the money rock.
If you get all 8 drops from the money rock, you'll collect just shy of 16000.
Worth looking for even if you don't need any normal rock items.
If you hate where a rock is placed, eat a piece of fruit and hit it with your shovel to destroy it. A new rock will appear in a random spot tomorrow.
Conversely, be very careful around rocks when your fruit meter is above zero! To be safe, dig up and replant some trees until you run out of fruit juice (or use a toilet, if you have one; it's a kind of furniture).
social interaction
If you talk to villagers and send them postcards, they'll become friendlier and you'll earn the ability to give them items — it appears as the second option when talking to them. You can give them clothing and they might wear it!
Try visiting villagers when they're at home. If they're in the middle of crafting, you can talk to them to get a copy of the recipe they're using.
If you neglect villagers, they might move out.
I don't understand villager behavior very well! Or real people behavior!