If they do not have any Sat nite for the next 50 weeks, then ML should revised their T&C. I would appreciate if it says "only Sun - Thurs redemption". Otherwise, it does sound like a bait-&-switch.
I wonder if Ritz categorized their hotels and maybe these cert will just never work for a weekend in top tier resorts, like Hawaii, or NYC. It'll be interesting to find out if any hotels anywhere will take the cert for a Sat nite for the next 50 weeks.