"Snow was swirling against the icy windows once more; Christmas was approaching fast. Hagrid had already singlehandedly delivered the usual twelve Christmas trees to the Great Hall; garlands of holly and tinsel had been twisted around the banisters of the stairs; everlasting candles glowed from inside the helmets of suits of armour and great bunches of mistletoe had been hung at intervals along the corridors. "
—Description of the month of December at Hogwarts[src]
December is the twelfth and last month of the year. It's one of the months of the year with thirty-one days.