I either want to set up a data validation formula, or do something
similar to IF(ISBLANK()) kind of thing.

I want a cell called "week starting date" to be locked to only
Saturdays (or a numeric test where I can choose which day gets tested).

So the user would choose the 'week starting day' (or number) in one
cell, and then the formula would take that choice and validate entries in
another named cell.

Or simply raise a flag (message) if it isn't starting on a Saturday.

This makes my timesheet calendar begin with the right day first and
date the subsequent days correctly.