You can book a Candlelight Processional dining package for any day that they have performances. The performances usually start right after Thanksgiving and continue to around Christmastime. There are three performances daily..something like 5:30, 7:00, 8:15..in that neighborhood anyway. Then, once you are able to start booking the packages, you can call Disney Dining to book your ADR. You are not able to apply this package to an existing ADR though.
It is not neccesary to book a CP package in order to see the show. Booking the package allows you to enter the show area earlier and be able to count on getting a seat. If you choose to 'gamble' on getting in without a package, you can wait in the line of standby guests. I have seen guests in those standby lines up to three hours before a show though. You can stand outside the theater and watch/listen to the show. I always book one package each trip in Dec, then I try to catch another performance on the fly so to speak on a different night. I get some dinner in the American pavilion and sit and eat as the performance starts.