So even though the rewards calendar only goes though Dec 31 if I buy a room reward and go to book it I'll have dates available into the new year?
I tried but it wouldn't let me purchase the reward because I have a reward room booked for later this month. will try after that trip to book a reward for my April trip.
Are you trying to buy the same reward you are using later this month? You may need to wait for a new batch of rewards before it will allow you to buy the same one twice.
You will be able to buy another MGM hotel reward 30 days after your next stay. Keep in mind you won't be able to buy the same room reward again unless they have released a new batch (i.e the price has changed).