At that time, Thomas, one of the Twelve, called the Twin, was not with them when Jesus came. The other disciples therefore said to him, We have seen the Lord. But he said to them, Unless I see in His hands the print of the nails, and put my finger into the place of the nails, and put my hand into His side, I will not believe. And after eight days, His disciples were again inside, and Thomas with them. Jesus came, the doors being closed, and stood in their midst, and said, Peace be to you! Then He said to Thomas, Bring here your finger, and see My hands; and bring here your hand, and put it into My side; and be not unbelieving, but believing. Thomas answered and said to Him, My Lord and my God! Jesus said to him, Because you have seen Me, Thomas, you have believed. Blessed are they who have not seen, and yet have believed. [Gospel for the Feast of Saint Thomas]
The Feast of Saint Thomas the Apostle has traditionally been celebrated on December 21st in the Latin Church: it is a great feast of a great apostle, and it is held on this day because it is considered the date of his martyrdom. It has been celebrated for so long on this day in the Western Church, that Saint Thomas of Canterbury himself -- whose martyrdom we will celebrate shortly, during the Octave of Christmas, and who made the name Thomas, previously not very used in English, great in England and in English-speaking nations -- was named after the Apostle, having been born on this feast day: December 21st, 1119 (or 1120). Unfortunately, this is one of the feasts (of an Apostle no less) that the committees that created the new mass of Paul VI out of nowhere decided to move elsewhere in their customarily careless and slapdash way due to their own pet historical or pseudo-historical prejudices.
The Syro-Malabar Church -- one of the "sui iuris" churches of the Catholic Church, and the main Eastern-rite Catholic Church in India, the land where the great Thomas died and is most highly venerated -- still celebrates this most eminent Apostle with all Traditional Catholics of the Latin Rite on this day. It is a strong reminder of the unforgettable profession of Faith in the Divinity of Our Lord Jesus Christ in the days nearest to His blessed Nativity. As Dom Guéranger said: