If it's Christmas week you may need to book a couple of days ahead to get a specific time. For the rest of December you can book the day before and get what time you'd like if you stop by one of the locations in the parks/resorts before midday. I wouldn't tie myself down to a specific time too early because everyone's plans change at short notice on vacation.
My only comment would be FIND the time to do it early in your stay, not leave it to the end or last day of your vacation. That will allow you to get the best use from any "incentives" like VIP fast passes you may get but more importantly it will give you the flexibility to take a couple of days to do any last minute research of the other resorts or think over any questions you may have before you sign up. The number one regret posted on the poll about membership on these boards was "we didn't join early enough" and the most regular reason was people kept putting off taking a tour to the end of the vacations and then folks were left with the choice of taking the kids to their favorite character/ride/park or taking the tour. 99 times out of 100 the tour is what gets put off until next trip. It's only in a couple of years time they realize that decision cost them thousands of dollars by delaying their membership.