Pick at for a precise moment or meeting point: I'll meet you at 5 PM. Pick in for longer periods, spaces you're inside: We moved here in 2026.
Saying you're somewhere? I'm at the bus stop pins the spot; She's in the kitchen means inside.
Pick by what you mean, not just the noun
Writing a meeting time? You'll use at when the phrase names a precise moment: The call starts at noon. Choose in when you're naming a stretch of time: The course begins in August.
Planning around a season or year? Write in winter or in 2026, because those name long periods. If you narrow it to an exact moment, switch: The train leaves at 7:15.
Giving your location to a friend? I'm at the door. I'm in the basement.
The choice comes down to how you're picturing the time: at marks a precise moment, so you'd write The call starts at noon, while in marks a longer stretch, as in The course begins in August.
When the same place takes both
Meeting someone at a named spot? Use at when your sentence pins down where they'll find you: I'll wait at the school. That treats the school as a point on their route.
Describing something inside the space? Use in when you mean the area within: The library is in the school. Same building, but now you're talking about its interior.
Neither choice is wrong. They answer different questions: at answers where to meet, in answers where something sits inside.
Naming a large venue as a meeting point? It still takes at: Let's talk at the conference center. Point of assembly beats physical size.
Pointing to something inside that venue? Switch to in: There's a cafe in the conference center.
Writing about a day part? English says in the morning, in the afternoon, and in the evening: The office is quiet in the evening. When you name night, it switches to at: The office is quiet at night. Ordinary use prefers at night over in the night.