in, on,at +time and date

in + month or year- In March, In 2003
on + date (with the year or without it) or day of the week- On April 2, On March 3, 1999, On Saturday
at + clock time, midnight, noon- At 3:30 p.m., At 4:01, At noon
Remember also...
in + season- In the summer, In the winter
in + morning, afternoon, evening- In the morning, In the evening
at + night- At night

a piece of cake= super fácil