Change formulas to include additional cells as I fill them in?

Hi there,

This is my first post for help. Hopefully I'll get all the information in one shot for those willing to help me.
Thanks in advance!

I keep a spreadsheet with yearly sales data that I update each week with the new sales.

What I do now:

I update two of the three formulas at the bottom of the page each week to reflect and include the new week's worth of sales.
The first formula adds up total sales (nothing needed here)
Second formula adds up the sales from last year up to the point where the current week(this year) is and calculates the difference. (Work needed here)
Third formula calculates the percentage difference from the current sales to the same sales up to the current week of the previous year. (Work also needed here)
(Sorry if that is confusing.)

I have attached a spreadsheet to show exactly what I'm attempting to do.

I appreciate any help you can offer. Thank you.

ExcelHelp.xlsx

Thanks everyone! I will implement the help and see if I can get it going!