CP is the Candlelight Processional held in Epcot in World SHowcase during the holidays. I believe it starts the day after Thanksgiving. The dinner packages go on sale late summer, if I remember correctly. You don't need a ticket to get into the show (it's held in the theater across from the American Pavillion) but you do have to wait in a longer line until the folks with the dinner packages get in. They pay a little more for their dinners at selected restuarants in Epcot/WS and then get somewhat better seating. Lots of people just stand outside the back of the theater and watch from there.
As far as what CP is....it is the telling of the Christmas story, by a guest speaker. These speakers change over the course of time. As a part of the story is retold, there is a break and the beautiful choir and orchestra play/sing a song appropriate for what was just read. It is all quite wonderful and beautiful. It is my dd's favorite thing about going to WDW during the holidays.
