Just like last year, and not to let the rest of the competition reap all the benefits, Valve has started its massive Christmas sale. It can't be missed since the frontpage has been transformed into a massive ad, that is, there's no regular Steam page. In addition to savings up to 80% in most of the games in the store, each day until the 3rd of January, a set of daily deals will be published, with even greater savings over the Christmas discount.
Word of advice: Do not buy anything else except for the daily deals you might be interested in, since it may be possible that one day you find out Valve is offering such game for a lower price. Considering that some of the games offered during the past Thanksgiving Sale are available for much lower prices, I would not rule out such possibility. Heck, even I feel a little scammed after seeing some games I bought not that long ago now available for almost nothing.
Of course, once you know it's the last day of the sale, buy anything else you want without remorse!