Please help

trying to get an if formula to work based on work schedule dates.
If cell a1 is a certain date eg. 20-dec-06 and is between start date and end date then what is the pay#?


Cell A1 = Date (this will change)
B1 = pay # (26 pay dates)
C1= begin date
D1= end date

example
A2 = 20-dec-06
B2= 1
C2=10-Dec-06
D2= 23-dec-06


Thank you,

Iona