I've seen a few posts along this line, but I haven't figured out a good
solution. I want to sum by weeks of the year:

=sumif(weeknum(a1:a100), "=1", b1:b100)

But this doesn't work since you can't have weeknum there. I want this for
every week, and I'd like to make it more general, so I don't really want to do

=sumif(a1:a100, "1/1/2007" AND < "1/8/2007", b1:b100)

for each week of the year.
Is there a general way to do this?

Thanks