Hey, Tom-
I'm not sure I'm following your question, so let's see if I have it
right. You have historical data for 110 weeks of sales, which I mocked
up in a spreadsheet by using May 1 2005 in the far-right column.
Column A (in my sprdsht) is labeled 3/30/2003; column #110, or col DF,
is labeled 5/1/2005. I populated row 3 in each column with simulated
data.

If you need an average of the date in question and the prior 51 weeks,
could you simply write a formula for it and copy it into each column?
For instance, in my mockup the formula for the May 1 2005 52-week
average is =AVERAGE(BG3:DF3). Would your application be solved by
copying that formula into all the previous columns? This assumes your
columns are contiguous (no gaps) and fully populated. Would this do it
for you, or do I still not get it?

Dave O