First Quarter -- January, February, March
Second Quarter -- April, May, June
Third Quarter -- July, August, September
Fourth Quarter -- October, November, December

The above is based on a calendar year; if it is based on a fiscal year, one would need to know when the fiscal year starts.

For example, the Commonwealth of Pennsylvania's and many other governments start their fiscal year on July 1. Thus, the first quarter would be: July, August, September and so on.

For the younger forum members, remember this because you'll need it later on for different classes.