Monthly Average From Daily Data

I have data in the following format (columns A & B):

Code:
Day Value
1/1/2002 2.1
2/1/2002 4.8
3/1/2002 0.1
" "
" "
" "
31/12/2011 5.2

I'd like to be able to write a formula so that I can get the daily data averaged over each month, e.g so that I have something like:

Code:
Day Value
Jan 2002 3.3
Feb 2002 4.3
Mar 2002 3.1
" "
" "
" "
Dec 2011 4.2

I'd like the data in monthly averages so that I can graph them. I have read about array formulas and averageif statements, but I can't seem to combine them to make them work.