The feast of St. Thomas of Canterbury is of semi-double rite. The liturgical colour of the day is red. St. Thomas of Canterbury, or St. Thomas Becket, fell foul of the political machinations of King Henry II and was slain by the King's soldiers in Canterbury Cathedral on December 29th, 1170. The liturgical celebration of his feast entered Western calendars almost immediately after his canonisation. Apart from Vespers the Office is taken mostly from the Common of Martyrs.
At Mattins the invitatory is Regem Martyrum Dominum, Venite adoremus and the Office hymn is Deus, tuorum militum sung with the Doxology of the Incarnation. The antiphons and psalms are taken from the Common of a Martyr. In the first nocturn the antiphons In lege Domini etc (not doubled) are sung with psalms 1, 2 & 3 and the lessons are from the Common, A Mileto Paulus, with their responsories. These are the same as those in the post-Clement editions. In the second nocturn the antiphons Filii hominum etc are sung with psalms 4, 5 & 8. The lessons are hagiographical and are followed by responsories from the Common. The lessons are the same as those in the modern books. In the third nocturn the antiphons Justus Dominus etc are sung with psalms 10, 14 & 20. The lessons are from a homily of St. Chrysostom on St. John's Gospel followed by the responsories from the Common. The lessons are the same as those found in the modern editions. The Te Deum is sung.
At Lauds the antiphons Qui me confessus fuerit etc are sung with psalms 92, 99, 62-66, Benedicite and 148-149-150. The Office hymn, sung with the Doxology of the Incarnation, is Martyr Dei, qui unicum. After the collect of the feast commemorations are sung of the Octave of the Nativity, the Octave of St. Stephen, the Octave of St. John and the Octave of the Holy Innocents.
At the Hours the hymns are sung with the Doxology of the Incarnation and the antiphons of Lauds are used in the usual order. At Prime (Pss. 53, 118i & 118ii) the lectio brevis is Justus cor suum.
At Mass the Gloria is sung, the second collect is a commemoration of the Octave of the Nativity, the third collect of the Octave of St. Stephen, the fourth collect of the Octave of St. John and the fifth collect of the Octave of the Holy Innocents. The Creed is sung and the preface and communicantes are of the Nativity.
Vespers are of the Octave of the Nativity but from the chapter of the following Sunday within the Octave of the Nativity. The chapter is Fratres, quanto tempore and the Office hymn Christe redemptor omnium. After the collect of the Sunday commemorations are sung of St. Thomas, the Octave of the Nativity, the Octave of St. Stephen, the Octave of St. John and the Octave of the Holy Innocents. At Compline the Dominical preces are omitted.