Holiday Sale 2014 is a special set that was made available during the 2014 Winter Holiday Sale, which ran from December 18 to January 2, 2015. It started with the Auction event, from December 15 to December 18, which brought 2 special badges rewarding actions related to the auction system. The event also brought a new special currency, Gems, which can be obtained by recycling cards, emoticons or backgrounds. Gems are used to bid on the auctions, and can also be used to craft booster packs.
Crafting a badge from any game gave you a card from this special set during both parts of the event. Unlike most badges, which can only be crafted a maximum of 5 times, this one had unlimited levels.
All Holiday Sale 2014 cards will expire on January 2, 2015 at 11AM PST. They should be used, sold or recycled for Gems before then as they will vanish at that point.