If you can put a column in your data list that calcs the week number you
could use this formula:

=WEEKNUM(B1,2)

That function tells you what week of the year the date is in. In this case
the 2 in the forumula tells the function that my weeks begin on Mondays.

Then you could just use Excels automatic subtotals to put the subtotals in
for you:
DataSubtotals

I hope that helps.

Regards,
Ron