I am struggling to come up with an elegant formula solution to
accomplish the following task...

I have a schedule with people working 21-7 (21 days on and 7 days off)
and 10-4 (10 days on and 4 days off)schedules. By defining either of
these schedule options in one column and the day number of their
rotation (say, the schedule is starting on day number 3 of a 10 day
rotation) I want to create a formula to complete the rows of a
schedule to show "working" or "off" based on just these two
variables.