immediately! constantly! every day 'til you get what you want (or are willing to settle for)!
seriously, i'd call asap, and if what you want is not available, i'd call every day, even multiple times a day because cancellations happen regularly and as soon as they are cancelled they are thrown back in the "bank"
also, 48 hours before you want the ressie, if it's one that requires a credit card, is a good "window!"
just to make calling a bit easier, here is the toll free number 1-800-828-0028
GOOD LUCK!