Help - How do I create a formula that allows me to add up weekly totals?

Hello,

I apologize for my lack of knowledge concerning formulas, but I'm currently attempting to create a document in order to keep tracking of finances etc... In the document, I have a list of my daily expenses:

excel help 1.jpg

I then have a separate table, in which I'm adding up the weekly totals:

excel help 2.jpg

Up until now, I'd attempted "SUM (L3:L9)", but when I drag the box down, instead of become "SUM (L10:L16)" "SUM (L17:L23) etc (thereby adding up a weekly total), it just moves in the sequence "SUM (L4:L10)", "SUM (L5:L11)", =SUM (L6:L12) and so on...

I realize I'm probably supposed to be using something other than SUM, but I'm unsure as to what... If anyone could help it'd be much appreciated. Thanks a lot!