Yes the suggestion about booking online, and then calling to revise the reservation is a good one. I've done this multiple times with both Marriott and Ritz. In particular, the Ritz resorts want to try to slot you into Sat-Sat 7 day minimum stays during holiday periods. I've booked Sat-Sat, and then called the 800# to change it to a shorter slot without any problem. But if you try booking the shorter stay online, it shows as sold out.

Don't call the hotel direct; call the 800#.