It depends on what you're referring to. Are you referring to in and on as prepositions of time or prepositions of location?
If you are using them as prepositions of time, here is how I teach my adult students:
Use in for longer periods of time: in the future, in the morning, in March, in the spring, in 1492
Use on for days and dates: on April 15th, on Saturday, on the first day of Christmas, on the 30th
Use at for clock time and very short times: at 8:00, at midnight, at present, at this moment, at quarter-past three, at the same time every day
Hope that helps. :-D