The Candlelight Procession is being held this year on December 6th and 7th. It is a ceremony where CM's in choir robes walk with lit candles up Main Street and at Town Square (near the Train Station) they have a large stage set up and there is Christmas Carols and the Christmas Story is read from the Bible by a famous person (this year, John Stamos). There are two shows each day (forgive me for not knowing the exact times, but I think they are at around 3 and 7 pm). There were dinner packages that were sold on the 18th (I'm sure they are sold out already), that include dinner at a resort restaurant and provide an assigned seat for the show. People without paid packages are welcome to find standing spots on Main Street to view the show.
This event brings a lot of extra people into the parks on these two days, but you probably won't notice a huge difference in crowds than any other weekend in December. The important thing is to avoid Main Street during the performance times if you are not planning on trying to view it. I would personally head to DCA during the afternoons on these days in order to avoid the crowds.
Hope that helped.
