The most commonly missed deadlines in your first 90 days in Japan
Been meaning to write this up for a while. Moved to Japan a bit ago and got a couple of these wrong myself, so figured I'd save someone else the trouble.
The 14 day address registration thing catches almost everyone. You've got 14 days from actually moving into your place to register the address at your local city or ward office. Doesn't feel urgent when you're still unpacking boxes and jetlagged, but it's tied to your residence card and My Number, and if you drag your feet on it you'll hit weird snags later trying to open a bank account or sign up for a phone plan since they all want proof of a registered address. And it's not a one time thing either, if you move apartments again later the clock resets and you have to do it again.
Health insurance is the other big one. If your job isn't putting you on their insurance, you need to sign up for National Health Insurance (kokumin kenko hoken), ideally same trip to city hall or right after. A lot of people put this off thinking they'll deal with it once they're settled in, but the premium gets backdated to your enrollment date anyway, so waiting doesn't save you anything, it just means you're stuck without a card if you actually need to see a doctor in the meantime.
Also, pension enrollment. It's easy to just not realize you're supposed to be paying into National Pension since nobody really explains it clearly at the counter. Doesn't bite you immediately, but it matters a lot later if you're thinking about permanent residency down the line since they look at continuous payment history.
And then there's just keeping your residence card up to date whenever anything changes, address, name, visa status. People assume this updates automatically somehow. It doesn't, you have to go do it.
None of this is complicated once you know it exists, the annoying part is nobody tells you upfront and the city hall visit moves fast.