I think last time it was a developer sale for 1 developer for the duration of it.
Edit: Guess I'm remembering wrong:
The GOG.com Christmas sale kicks off on 8th December.
The headline act is The Witcher 2, which will be bundled with the Enhanced Edition of Witcher 1 for £14.99 - a saving of 40 per cent (not to mention the free game).
This Witcher 2 sale starts at noon on Thursday (8th December) and expires shortly before midnight on Saturday (10th).
GOG.com will give away more than 400 game codes for various old PC games on Saturday (10th) and Sunday (11th). Included in this giveaway are Master of Orion 1 and 2, and Realms of Arkadia 1 and 2. There will be "lots" of codes given out for games costing $5.99 and $9.99, too.
On Monday (12th), GOG.com will host a 48-hour giveaway of Empire Earth: Gold Edition. Anyone with a GOG.com account can claim the game for free.
Monday also marks the beginning of the "50% Off Holiday Celebration" sale. This will apparently span "virtually every game in the catalogue", and lasts through to 2nd January 2012.