I have a spreadsheet with three tabs.

Tab 1 needs to perform summary calculations
Tab 2 & 3 are ODBC connections with source data from other files.
Tab 2 is a sql query that is a cross tab with columns 1-4 representing row
data and then the remaining columns are employee ids with a productivity
value in their column. Not all columns have the same value as they correspond
to a job/service performed on a specific date.

My problem Tab 1, column 2 contains "completion dates from 1/1-current and
refreshes based on tab 2 each time it opens. I need a lookup formula that
will read the single date column and say if date begins with December, then
total the sum of tab 2 column "X" (for corresponding December values only)
and divide by the (total of tab 3 column (X) for the same time frame, and
then do it consecutively for all months. I've tried as a lookup and vlookup
and neither seem to be coming out right.

Any suggestions are appreciated: