The Chair of Unity Octave ("Unity Week") starts on Tuesday January 18 and ends on Tuesday January 25.
This observance was begun by Anglo-papalists in the early twentieth century specifically to pray for the Unity of all Christians in communuion with the See of S Peter and S Paul. It was encouraged by a succession of Roman Pontiffs and endowed with indulgences (see below).
At a time when PF has launched a relentless attack upon Tradition within the Latin Church, it is surely even more important to pray for Unity. Surely, in the front of our minds ought to be, not the old 1960s-style Ecumenism of ecclesiastical bureaucrats from different ecclesial bodies cosying up to each other, but the urgent need for Unity to be restored within the Latin Church herself.
The Chair of Unity has itself been converted into a sign and instrument of disunity: in this dreadful crisis, may God keep us and guide us.
Before the 1960s, January 18 was the Feast of the Chair of S Peter at Rome (while February 22 celebrated his Chair, that is to say, his episcopate, in Antioch). The Feast of the Conversion of S Paul on January 25 still survives, even in the Novus Ordo.
In the Good Old Days, the Wantage Sisters ... who now comprise our Ordinariate Sisters ... the praying heart of the Ordinariate, as our Ordinary puts it ... used to publish an annual ORDO "in strict accordance with the Use of the Western Church". This was widely used both in Anglo-Papalist churches and in Anglo-Catholic churches generally. The latest one was probably that of 1969, which I still have. Before January 18, the following information is printed:
CHURCH UNITY OCTAVE BEGINS
Ad lib, during the Octave: one 2cl Vot M For the Unity of the Church. Cr (on Sunday only), Common Pref (pref Trin on Sunday). P[urple]
This will undoubtedly have been lifted from what was authorised for Roman Catholics in England, Scotland, and Wales on the very eve of the liturgical alterations of the late 1960s. What it means is that it is lawful to say daily one Mass of the Votive for Christian Unity (Ad tollendum Schisma if your Missal, like mine, is pre-1962; but the texts are the same in the 1962 Missal) on the Sunday within the Octave (even if it be Septuagesima); and also on each of the weekdays, because they are all (even the Conversion of S Paul) days occupied by III class feasts and so admit Second Class Votives. No Gloria, of course. Only one Collect; Secret; Postcommunion; is said ... in other words, no commemorations.
My own practice is to start the Octave with a (perfectly legal) Votive Mass of the Chair of S Peter on January 18 (Mass as on February 22 except that the Alleluia is said; the colour is white) and to conclude with the Mass for S Paul on January 25. It was the idea of linking up the two Roman Apostles which gave rise to the Octave.
Alleluia for the Chair of S Peter outside Lent and Septuagesima: Alleluia, alleluia. Tu es Petrus, et super hanc petram aedificabo ecclesiam meam. Alleluia.
I have thought it worth while providing this information because I do not think it is in the available Authentic Use ORDOs in Latin, English or French.
In the current Encheiridion: Plenary under the usual conditions for a Catholic who shall have taken part in any functions in the week; and shall have been present at the conclusion of this week (i.e. on 25 January). Partial for whosoever shall have devoutly recited an approved prayer for Unity.
ORDINARIATE 'DIVINE WORSHIP' MISSAL
The same Mass for Unity, of course, is provided for use in Liturgical English in the Ordinariates. The rubrics make clear that it can be said on any day except Solemnities, the Sundays of Advent, Lent, and Easter, All Souls, Ash Wednesday, Ember Days, Rogation Days, weekdays of Holy Week and of the Easter and Pentecost Octaves. Such votives ARE allowed BUT ONLY FOR "a real necessity or pastoral advantage" on Obligatory Memorials and the weekdays of Advent, Christmastide, Lent, and Eastertide. Pretty permissive, eh?