It is actually not an official
Disneyland event, and is not listed on the main schedule. Disney does offer dining packages to get some reserved seats for this event (once again, no details yet). It basically started as a private event set up by the CM's (employees) as a thank you to the park sponsors and other main folks that helped out the park. They allowed the public to watch if there was any leftover space, and it has grown into a big event.
Currently, guests are allowed to line up and watch behind the reserved seats at no cost. Shows are offered twice a day, around 5 and 8 PM.
It does force the parades that weekend to be offered earlier than normal (you get one in the morning), the Railroad operates for only part of the day and that the Main Street Station is closed. Also the traffic flow in and around the Town Square and Main Entrance is very heavy both evenings. Fireworks will be offered as normal at 9:25 PM (weather permitting).
The CM parties are scheduled for Monday Dec. 3rd and Tues Dec 4th, Disneyland will closes at 6 PM both nights, so the parade is held earlier, and the Fireworks are offered at 5:45 PM (in December it is already dark by then). DCA will remain open a few extra hours to allow guests to remain after 6 PM.