Applying a formula only to the first date in a month

I have a row of pay dates, B7-B36, each two weeks apart, e.g. 5/4/12, 5/18/12, 6/1/12, etc. I also have a row of values (E7-E36) that I want to pull from another cell (B3), but I only want the value to show up on the first pay period of each month. What formula can I use so that the calculated number only shows up if it's the first pay period of the month, and otherwise shows "0"?

Thanks!

Terrell