The religious festival of Christmas is twelve days long - it starts on Christmas Day (and not before) and ends on Jan 5th (Twelfth Night).
Some traditions include the Epiphany season, which extends to Candlemas (Feb 2nd, forty days after Christmas Day), which commemorates Christ's presentation in the Temple. Shrove Tuesday can fall as early as the following day, but has not done so since 1818 and will not do so again until 2285.