Automate lookup formula to refer to sheet entered in a cell

Here's my situation that I need help with.

I have a file that has multiple worksheets (Apr, May for example). I currently use a lookup formula in B5 +LOOKUP(A5,APR!B2:B10,APR!A2:A10) to look at the unit code get the amount from the April worksheet. I regularly have to change the formula in B5 to use worksheets for differnent months, so I might change the Apr to May to get the correct amount. (I use the lookup formula as the units are in the column after the amounts, so the Vlookup won't work). I want to find a way to automate the formula so I can use a date field (A2) and have it find the correct information in the correct worksheet. I was thinking that I could use the text formua to identify the correct month, but I'm not sure how to integrate it into my formula. Everything I've tried so far, doesn't work. Can someone help?



DATE
(A2) 4/1/2011

UNITS AMOUNT
(A5) AR-650287 36,524.00 (B5)

Apr
TEXT(A2,"mmm")

Try

=LOOKUP(A5,INDIRECT("'"&TEXT(A2,"mmm")&"'!B2:B10"),INDIRECT("'"&TEXT(A2,"mmm")&"'!A2:A10"))