Holiday Sale 2015 is a special set that was made available during the 2015 Winter Holiday Sale. The cards initially became available on December 8, 2015, though were not craftable into a badge until the sale began on December 22, 2015. Unlike previous sales, in which the mysterious cards were all revealed at once when the sale began, the cards are being revealed one card per day to coincide with the Northpole Noir comic that accompanies the sale. Until the day that a card is revealed, the images is a blurred shadow version of the image.
There are 11 cards in the series, and you can earn up to three cards a day by browsing your discovery queue. You can also earn cards by crafting a badge from any other card set - doing so will give you one card at random in addition to the normal badge crafting rewards. Additionally each $10 spent from buying games will grant you one card.