The feast of St. Thomas of Canterbury is of 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, of course, with the Doxology of the Incarnation. The antiphons and psalms are taken from the Psalter for Thursday. In the first nocturn the lessons are the Incipit of the Epistle of St. Paul to the Romans. In the second nocturn the lessons are hagiographical and in the third nocturn the lessons are from a homily of St. Chrysostom on St. John's Gospel. The Te Deum is sung. At Lauds the antiphons and psalms are from the ferial psalter. The Office hymn is Invicte Martyr unicum, again sung with the Doxology of the Incarnation. After the collect of the feast a commemoration is sung of the Octave of the Nativity.
At the Hours the hymns are sung with the Doxology of the Incarnation but the antiphons and psalmody are ferial. At Prime 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 Creed is sung and the preface and communicantes are of the Octave 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 with a commemoration of St. Thomas Becket and of the Octave of the Nativity.
Following the 'liturgical books of 1962' St. Thomas is reduced to a commemoration in the fifth day within the Octave of the Nativity. The liturgical colour of the day is white. However, festal psalmody is used at Mattins and Lauds, as on the feast of the Nativity (this contrasts with the previous practice for third order Octaves when a double feast falls within them). Mattins is reduced to one nocturn of three lessons. At the Hours the hymns are not sung with the Doxology of the Incarnation, the antiphons and psalmody are ferial, at Prime the lectio brevis is of the season. Mass is of a day within the Octave, with Gloria, commemoration of St. Thomas (at read Masses only), Creed, preface and communicantes; of the Nativity. Vespers are of the Nativity without any commemorations.