The Feast of St. Hilary of Poitiers is celebrated today and is of double rite. St. Hilary was a bishop in fourth century Gaul.
Today, after yesterday's celebration of the Octave Day of the Epiphany, the season after the Epiphany really begins. The Suffrage of the Saints is resumed on all days of semi-double rank and below (so not actually sung today) along with the Dominical preces at Prime and Compline. At Prime the versicle Qui sedes ad dexteram Patris is resumed in the short responsory.
At Mattins in the first nocturn the responsories the Epistle to the Corinthians continues to be read but with the responsories from Monday belonging as they do to the Incipit of the Corinthians), in the third nocturn the ninth lesson is of St. Felix. At Lauds a commemoration is also sung of St. Felix.
At Mass, In medio, the Gloria is sung, a commemoration of St. Felix is made, the Creed is sung (as St. Hilary is a Doctor)and the Common Preface is used.
Vespers are from the chapter of the following feast of St Paul the First Hermit with a commemoration of the preceding Office of St. Hilary and a commemoration of St. Maurus, Abbot. As the feast day of St. Paul is not his die obitus the third verse of the hymn Iste Confessor is changed from Hac die laetus meruit beatas to Hac die laetus meruit supremos. In both the Breviary and Ordo this change is indicated by the letters 'mtv' - mutatur tertius versus.
In the 'liturgical books of 1962' the familiar pattern of a former three nocturn feast being reduced to a single nocturn of three lessons is repeated. The responsories from the Incipit of the Corinthians are omitted. St. Felix does not get a lesson at Mattins but is commemorated at Lauds. At Prime the lectio brevis is of the season rather than from the Common. At private Mass St. Felix is commemorated, the Creed is neither said nor sung. Vespers are of St. Hilary without commemorations. The hymn Iste Confessor is never changed and has as its third verse Hac die laetus meruit supremos always. The whole concept of concurrence of Office has been lost, largely due to the 1955 abolishment of first Vespers, a practice that reflected the truly ancient idea of the liturgical day beginning at Sunset.