According to tradition, St. Thomas the Apostle was martyred on December 21, and for many centuries the Roman Rite of the Catholic Church celebrated his feast on that day.
Thomas may seem a curious apostle to celebrate right before Christmas, but his example can be an inspiration to us, who sometimes doubt the power of the Christ Child at Christmas.
Here is a prayer from Dom Prosper Gueranger in his Liturgical Year that asks for St. Thomas' intercession during these final days of Advent.