The Feast of St. Thomas takes precedence over the feria, which is commemorated.
Proper Mass; Gloria; commemoration of the feria; Credo; Preface of the Apostles.
Matins: Invitatorium Regem Apostolorum Dominum; Hymn Aeterna Christi munera; psalms and antiphons from the Common of the Apostles; Te Deum
Lauds: Antiphons from the Common of an Apostle with Sunday psalms; Hymn Exultet orbis; commemoration of the feria. Nota bene: For the commemoration of the feria, the Benedictus antiphon is proper to December 21 (printed in the proper section after the III Sunday of Advent): Nolite timere * quinta enim die veniet ad vos Dominus noster.
Hours: Thursday psalms and antiphons
Vespers: Antiphons and psalms from the Common of an Apostle; commemoration of the feria with antiphon O Oriens (O Antiphons for Advent)
Compline: Of Sunday