Hello @Don120! Welcome to Airbnb and the forums.
I use Saturday as my check in and out day during peak season, and I'm completely booked for the summer months. As a guest, it's what I would prefer: I think it lets you make the most of a one-week vacation. You can travel Saturday morning on the day of check-in and still have a bit of time to do stuff on the Saturday afternoon. On the other end of the vacation, you can go home on Saturday and you still have all day Sunday for recovery, unpacking, etc. On a pracical note for me, as a part-time host who works Monday to Friday 9-5, it also means that I'm available if there's a problem at check in or check out and I get to personally check my listing between guests, which is really important to me.
A lot of my competition does Friday to Friday or Monday to Monday so they can also offer half-week mini-breaks (bookings Monday to Friday/Friday to Monday). I don't do that, and I really don't like the Monday to Monday option.
I would also check to see what the hosts around you are doing. Checking on your competition never hurts.