Hi Andrew,
You can use B1700690 - F0007 Get Day of Week BSFN, you pass the branch plant and date, this bsfn check branch/plant calendar P00071, and return day of the week (for you is not relevant) and type of day. You check if this value of type of day ('W' Work, 'H' Holiday, 'V' Vacation, 'E' Weekend), etc.
I hope it help you.
Regards,
Alfre.