As far as I am aware, the Deer Dance is only done in the Toledo district, which is in the South of Belize. The Joncunu dance is performed in Dangriga, which is also south, though not as far as Toledo.
At the beginning of December we have a lighted boat parade in San Pedro. There is a Christmas decoration competition and that means that it is well worth going around the island at night to enjoy the lights. There is a very well attended Catholic Church in town and they do hold Christmas midnight mass. Many restaurants do a fabulous feast for Christmas, so you will find that you are spoiled for choice.
Santa Claus usually finds his way to San Pedro by boat, which is always a fun time.
You'll have a great time.