The prepositions in, on, and at can be used to indicate time and place. Notice how they are used in the following situations:
PrepositionTimePlace
InYear, Month,
In 1999, In DecemberCountry, State, City
In Japan, In Utah, InTaipei
OnDay, Date
On Saturday, On May 1Street
On Main Street, On 1st Ave.
AtTime
At 8:00, At 7:30Address
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. Examples:
A: Where's your office? B: In Taipei, Taiwan. A: Really? What part of Taipei? B: It's on Chung Shan North Road. A: I know that area. Where exactly is it? B: It's at 105 Chung Shan North Road, next...