I need to set up a series of recurring dates in excel for various
clients. They will be recurring in increments of every 2 weeks or every
3 weeks etc. I'll want to use it for easy access to the date I need to
schedule the next appointment since I'll do that at the end of every
month for the following month. So if my June appointment for client X is
on the 19th the next will be July 10th then 17th and so on....how do I
calculate that? Thanks!

Dates in Excel are just whole numbers. Add 14 for two weeks later; 21 for 3 weeks later; etc.

Maybe I'm missing the point, though. The question doesn't seem to specify when to use how-many weeks.