A1. at for the time of day, and at night, at the weekend, at Christmas, at the same day, etc.
A2. on for days and dates
A3. in for longer periods (month/ year/ seasons), and in the morning but on Friday morning; in a few minutes/ in six months etc.; in ~ to say how long it takes to do something
A4. We do not use at/ on/ in before last/ next/ this/ every
A5. In spoken English we often leave out on before days (Sunday/ Monday etc.)