The Christmas Parade is taped over 2 days (Fri. & Sat.) during the first full weekend in December, which would be Dec 5&6 this year. I believe they also might tape Sunday morning as well if there have been any weather issues during the first 2 days. They do the castle show taping on Friday and the parade down Main Street on Saturday. We were there this past December during the parade taping but avoided going to MK during those days. We went on Sunday after the taping and had pretty small crowds all morning until about noon.
From my understanding of the taping itself, it is like just like any TV production where they will do things over and over again to get the right shot. As for tickets to the taping, I think they had a website to get tickets on a first come first serve basis. If you are on Main Street you are expected to come dressed for Christmas (hats, sweaters, etc.) and stay for the duration of the taping. After that you are free to spend the rest of the day in the park.