Naming a day or date, use on: The meeting is on Monday. A month, season, year, or general part of the day takes in: She was born in 1999. An exact clock time or festival takes at: Let's talk at three.
Choosing by what you're pointing at
Naming a day or a slice of one? Use on: The interview is on Friday afternoon. Your sentence points at a specific day, so on fits.
Naming something broader, like a month, season, or year? Use in: We're moving in July. You're inside a longer period, so in fits.
Naming an exact clock time or a festival? Use at: The show starts at eight. You're pinpointing a moment, so at fits. At night works the same way, even though it feels broader.
Naming where something physically is inside something, from a drawer to a country? Use in: The keys are in the drawer. Your sentence describes being surrounded on all sides.
Naming something touching a surface? Use on: Your notes are on the desk. Contact with the surface is what your wording communicates.
Naming a location as a point rather than an enclosed space, like a stop, a door, or an event? Use at: I'll wait at the bus stop. Your sentence treats the place as a pin on the map, not a container, so at fits, as in She's at the door or Meet me at the concert.
When the size of the unit settles it
The qualifier that settles it is precision: the more precisely you point, the smaller the preposition gets. Keep the same phrase and just narrow it, and the preposition shifts with each step: in the morning, on Monday morning, at six in the morning.
So when you're choosing, ask how small a slice of time your sentence names. A season or month takes in. A named day takes on. A clock time or a specific moment takes at, and that includes the fixed exception at night.
Try it on any time expression in your draft: name its smallest unit. If you can shrink the phrase further, the preposition shrinks with it. If it's already a single moment, at is as far as it goes.