Copying data from one sheet to another based on variable date range

Hello,

I have a sheet which I have managed to merge with my company's Oracle database. This sheet pulls sales numbers from multiple locations when first opened. Additionally, every day a new cell is updated and automatically generated from the previous day's sales. I am looking for a function that will pull weekly summation of data from a variable, daily changing, range.

For example:

Dates Sales

Day 1 $500
Day 2 $300
Day 3 $200
Day 4 $235

If I need to post my work so far, I am willing. Thanks in advance!