Christmas begins at midnight on December 24 (early morning on December 25). People should not start putting up Christmas decorations before Christmas Eve. Christmas lasts for eight full days and ends on January 1 - the Octave of Christmas. Christmas lasts until Epiphany on January 6, so your decorations should stay up, and the Christmas Antiphons are sung until February 1, so you can take them down at the end of January. Please do not put up Christmas decorations during Advent. Advent is the penitential season and includes the four Sundays before Christmas, so it starts around the end of November. Again, Advent is the penitential season, so nothing about Christmas should interfere with Advent except preparation - spiritual preparation for Christmas, confession every Saturday, extra prayer, daily Mass, etc. All of these are excellent preparations for Christmas, but don't start celebrating Christmas before midnight on December 25!