Counting Sundays

Relatively new to Airtable. I’d like to be able to calculate the total number of workdays in a given month for employees who work six days/week in a museum, so Tuesday through Saturday. My approach has been to take the total number of days in a given month using DATETIME_DIFF and then try to figure out a way to count just the Sundays in a given month and subtract these from that total.

Ideally, I could just define WORKDAY_DIFF parameters (in fact, it seems obvious to me that I should be able to, but I don’t see any reference to this/any way to do it . . . just the outdated(?) assumption that a workweek comprises one set of values).