Used with years (in 1998), months (in April/May), seasons (in summer/spring), centuries (in the 20th century) and parts of the day (in the morning, in the afternoon, in the evening) except: at night/midday/noon/midnight. It is also used with events that will happen at the end of a period (in a week, in one month, in a couple of minutes).
Used with days and specific dates (on Monday/Tuesday/Sunday afternoon/Christmas’ Eve/New Year’s Day)
Used with clock time (at 7 p.m), weekends and holiday periods (at Easter, at weekends, at Christmas) and points in time (at the end of the film, at the beginning, at present, at times/the time, at the moment)
We do not use any preposition before next (next summer) and last (last year).
Here are some exercises to practise.