The Sword Art Online - Extra Edition anime special, recapping the Aincrad and Fairy Dance arcs and including new footage, is now scheduled to run on December 31st on Tokyo MX. Looks like the original content is basically going to be a swimsuit episode...
In its original story, the characters accept an ALO underwater quest when they learn of Yui's wish to see a whale. Discovering that Suguha/Leafa can't swim, Asuna takes the girls to a real world pool for training. Meanwhile, Kirito is having mysterious meetings.
When the 13th light novel was released in August, author Reki Kawahara described plans for the end of year anime special. In the afterword, he explained that "Extra Edition" will be an abridge retelling of Aincrad and Fairy Dance, with some new footage included.
Author Reki Kawahara on the plans:
“I’d like to do a bit of promotional plugging on this page here. You probably already know this from looking at the book’s wraparound jacket band, but at the end of 2013 the Sword Art Online TV anime series will be getting a TV special.
Although the year-end TV special is fundamentally a compilation of the Aincrad and Fairy Dance chapters from the 2012 TV anime series, there will be some new scenes added.
It’s been almost a year since we’ve all been able to see Kirito move around in animated form on a TV screen, so please do watch the TV special.”