Christmas in Mexico is a celebration that spans several weeks, beginning on December 3rd with the Festival of the Virgin of Guadalupe, and ending on February 2nd with the Día de la Candelaria, with Las Posadas and Los Santos Inocentes in between.
Christmas trees are not traditional in Mexico but they are becoming more common because of the influence from the USA. The most important decoration though is a Nacimiento, or nativity scene. They are set up on December 12th and Jesus is added to it on Christmas Eve, and the Three Kings are added on the 5th of January.
On Christmas Eve there is a late mass called la Misa de Gallo (Mass of the Rooster), so called because a rooster is said to have crowed at midnight on the night Jesus was born. After mass there is a meal, traditionally roast pig Pig, but nowadays often turkey, and just a few small presents brought by Santa (another influence from USA) . Christmas Day itself is quiet, and usually spent with the family.
On the night of 5th January, children leave their shoes by the door and the Three Wise Men visit and leave the bigger presents. The following day there is more eating, including the Roscón de Reyes, to decide who will pay for the tamales at Candlemas!