Open the sheet where you want to change the week start day.
Find the week column on the Transactions sheet (likely to the far right).
Right click the column letter and choose insert 1 left.
Paste this formula into the header (row 2).
=ARRAYFORMULA(IF(ISBLANK($A:$A), "", IF($A:$A <> "Date", $A:$A-WEEKDAY($A:$A,3), "Alt Week")))
A few things to keep in mind about this ArrayFormula:
You still won’t be able to edit calculated values on an individual row. In other words, in this example, if you wanted to change the week, your only option is to change the data in column A.
Inserting new columns or changing the order of the columns can be problematic so make sure you insert the column in a good spot when you’re setting it up the first time.