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. To this day those following the Armenian Rite keep Theophany (the Nativity & Epiphany as one feast) today - a happy Christmas to any Armenian readers!
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 ommitted and the Office begins with the first antiphon Afferte Domino. In the first nocturn the lessons are from Isaiah. In the second nocturn the lessons are from a sermon on the Epiphany by St. Leo. In the third nocturn the usual invitatory psalm, Venite , is sung in the third nocturn, in a responsorial manner, with the verse Venite, adoremus eum: quia ipse est Dominus, Deus noster. A homily of St. Gregory is read in the lessons of the third nocturn. At Lauds the antiphons sung at Vespers yesterday, Ante luciferum genitus etc., are sung with the Dominical psalms.
At the Little Hours the antiphons from Lauds are sung with the festal psalms. At Prime 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.
Die Vigesima Februarii, erit Dominica in Septuagesima.
Nona Martii, Dies Cinerum et initium jejunii Sacratissimæ Quadragesimæ.
Vigesima quarta, Sanctum Pascha DNJC cum gaudio celebrabimus.
Secunda Junii, erit Ascensio DNJC.
Duodecima Junii, Festum Pentecostes.
Vigesima tertia Junii, Festum Sacratissimi Corporis Christi.
Vigesima septima Novembris, 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.
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. Tomorrow however...