need qualifier statement

Hey guys...

I have a weekly check sheet and the days of the week auto-fill from the
"week starting date" that I fill in in another cell. That works fine. I
gave the week starting date cell a name of StrtDate

My company's start date begins on Saturday, so if one enters the wrong
date in week starting date cell, the first day shown on the check list
area is not Saturday, and the person should see right away that they have
entered an incorrect date. I want to automate that check.

Would I simply place a formula in the data validation box for the
starting date cell that locks it to only Saturdays being valid entries?
If so, can you give that formula string? The data validations I saw for
dates did not allow such discreet dissemination, so I presume it is a
custom validation formula.