The feast of the Epiphany is a Double of the First Class with a privileged Octave of the Second Order. The Epiphany was kept in the East from the third century, or earlier, and spread to the West a century later. Originally, it seems, in the East at least that this 'manifestation' of the LORD was the same celebration as the Nativity.
The antiquity of the feast can be clearly seen in the structure of the Office of Mattins, unique to this day. The (historically later) Domine labia me, invitatory and hymn are omitted and the Office begins with the first antiphon Afferte Domino. Psalms 28, 45 & 46 are sung. The lessons in the first nocturn are from Isaiah. In the second nocturn the antiphons Omnis terra adoret te etc are sung with psalms 65, 71 & 85 and the lessons are from a sermon on the Epiphany by St. Leo. In the third nocturn the antiphon Venite adoremus eum etc is sung with Ps. 94, Venite , the usual invitatory psalm, in a responsorial manner. Psalms 95 and 96 are also sung in the third nocturn. The homily on St. Matthew's Gospel if from St. Gregory. The Te Deum is sung. At Lauds the antiphons sung at Vespers yesterday, Ante luciferum genitus etc., are sung with the Dominical psalms. The chapter is Surge, illuminare from Isaiah and the Office hymn is O sola magnarum urbium.
At the Little Hours the antiphons from Lauds are sung with the festal psalms. At Prime (Pss. 53, 118i & 118ii), in the short responsory, a versicle Qui apparuisti hodie is sung today and during the Octave, the short lesson is Omnes de Saba. The Doxology Jesu, tibi sit gloria, Qui apparuisti gentibus etc is sung at the hymns of the Little Hours.
Mass is sung after Terce. The Mass, Ecce advenit, is proper. The Gloria and Credo are sung. The preface and communicantes in the Canon are proper to the feast.
After the Gospel of the Mass the Moveable Feasts for the year are traditionally announced. The chant is very beautiful. The text is as follows:
Noveritis, fratres carissimi, quod annuente Dei misericordia, sicut de Nativitate DNJC gavisi sumus, ita et de Resurrectione ejusdem Salvatoris Nostri gaudium vobis annuntiamus.
Vicesima septima Januarii, erit Dominica in Septuagesima.
Tertia decima Februarii, Dies Cinerum et initium jejunii Sacratissimæ Quadragesimæ.
Tricesima prima Martii, Sanctum Pascha DNJC cum gaudio celebrabimus.
Nona Maji, erit Ascensio DNJC.
Undevicesima Maji, Festum Pentecostes.
Tricesima Maji, Festum Sacratissimi Corporis Christi.
Prima Decembris, Dominica prima Adventus DNJC, cui est honor et gloria in sæcula sæculorum. Amen.
At Vespers the antiphons Ante luciferum genitus etc are sung with psalms 109, 110, 11, 112 & 113. The Office hymn is Crudelis Herodes Deum. The antiphon on the Magnificat is Tribus miraculis.
In the 'liturgical books of 1962' the only textual changes today are the abolition of the Doxology in honour of the Epiphany at the hymns of the Little Hours and the duplication of antiphons at the same. The Octave and, as noted yesterday, Vigil have been stripped from the feast.