Dylan here and Nick is airing a new commerical for the finale and Viacom has put out a press release about Thriller Week.

A character is revealed to be returning in the commerical in some new footage.

New info in the press release includes: "A dedicated Avatar site will feature the new Game of the Week Rise of the Phoenix where Aang goes one-on-one with the Phoenix King, an Avatar game Tournament for Avatar gamers to go head-to-head, polls, trivia challenges, fun facts, as well as additional exclusive content."

Also: "Viewers can catch up on Aang's epic journey by tuning into Nicktoons Network for mini marathons of Avatar each day beginning Monday, July 14 through Saturday, July 19 at 2:00 p.m. The series will roll out in succession, starting on Monday with the series premiere and will culminate with a Nicktoons Network encore of the two-hour TV movie, Sozin's Comet, on Sunday, July 20 at 5:00 p.m. ET."

The press release offers this synopsis for the finale: "Book Three culminates with a two-hour movie, Sozin's Comet, on Saturday, July 19 when Aang's struggle with firebending becomes urgent as he learns he must confront the Firelord earlier than planned. Aang prepares mentally and physically for the attack as the comet nears, but when he finally faces the Firelord, will he have the heart to take his life?" 

Thanks to Jason for the commerical link!