We sailed on Ascent for the Christmas cruise this past December. This was the 4th time we had sailed at Christmas on Celebrity so we had a good idea what to expect.
It was impossible to find available times in Specialty Dining except for a few options at 930 PM (too late for us!). Beginning in early October, I tried booking online and I even spoke to Captain's Club. Everything was booked ... We wound up eating in SD only one night (Eden - and it was excellent, worth every penny).
As it turned out, the sailing had a number of large families - big groups of 25 or more, and since many of those groups had someone in a suite, they seemed to snag all of the reservations. We've never before encountered this and I don't know if it was because the ship was brand new, or sailing from FLL, or people are still wanting special travel experiences.
The downside was that we didn't get a chance to try any of the other SD restaurants. The upside was that we were always able to get a table in Blu the minute we walked in since so many people were dining with families in SD or one of the MDRs.
So my suggestion would be to find out exactly when you can reserve and book early. You can always cancel later if your plans change.