PREPOSITION IN, ON, AND AT

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.

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 to the bookstore.

C: When is the wedding?
D: It's in June.
C: What day?
D: It's on Saturday, the 25th.
C: What time?
D: It starts at 6:00.

Prepositions with articles and locations
When talking about locations, use at to indicate the general vicinity or area, and in to indicate inside the building, enclosed area, etc.
Example:

at the swimming pool (on site)		in the swimming pool (in the pool itself i.e. in the water)
at the post office/bank (general)		in the post office/bank (inside the building)
at the zoo (visitors, general area)		in the zoo (animals in their cages)
at school		in the classroom