The prepositions in, on, and at can be used to indicate time and place. Notice how they are used in the following situations:
Preposition
Time
Place
In
Year, Month, In 1999, In December
Country, State, City In Japan, In Utah, InTaipei
On
Day, Date On Saturday, On May 1
Street On Main Street, On 1st Ave.
At
Time At 8:00, At 7:30
Address At 815 East Main Street
In many languages, there is only one preposition for the above situations. In English there are three. Just remember that in usually indicates the "largest" time or place, and at usually indicates the "smallest" time or place.