YGO TCG Advent Calendar 2019 - Rules of Play says:

Coming this holiday season, the Yu-Gi-Oh! TRADING CARD GAME Advent Calendar is here to honour the old-time tradition of counting down the days to Christmas, in a way fit for a Duelist! This year’s Yu-Gi-Oh! TRADING CARD GAME Advent Calendar is more compact and double-sided, with 12 doors on each side, but still packed to the brim full of holiday-themed cards to add to your collection and power up your Decks.