prepositions in time phrases

at
clock times, points of time in the day (at midnight)
weekends and holiday periods
points in time (at the moment)

in
the main parts of the day (in the morning)
months, years, seasons, centuries
events which will happen at the end of a period (in a couple of minutes)
at a point in time during a period (in the beginning)

on
specific dates and days (on Tuesday)