Major solemnities are sometimes preceded by a vigil office, and sometimes extended through an octave
As Pope Alexander III, who began to rule on the year of Our Lord 1159, says (Extra, De feriis, chapter II): “Although it is written, From evening until evening you shall celebrate your sabbaths, nevertheless the beginning and end of feasts must be considered according to their kind and the custom of the several regions. And just as the importance of the day demands that it be begun earlier and ended later,” the regulars, following the authority of Sacred Scripture, place the start and end of feasts from Vespers to Vespers. But based on their importance they precede them earlier with a vigil office and extend them longer with an octave office. Let us therefore discuss these two.
Vigil offices precede feasts of Our Lord, two feasts of his mother (namely the Assumption and, following a constitution of Gregory XI promulgated when he returned to Rome, the Nativity of Mary), and the days of John the Baptist, Laurence, and, as Innocent III says in De observatione ieiunii chapter 2, “the vigils of all the apostles are to be celebrated with a fast except the vigils of the apostles Philip and James and John the Evangelist, because the feasts of the former pair are celebrated within the solemnity of Easter, and the latter within Christmas.” As Alexander III says in De verborum significatione, chapter Quaesivit, “on the Vigil of Blessed Matthew, unless it fall on a Sunday, fasting is observed.” Vigils on which there is fasting consequently also have a Mass. In the antiphoner which I brought from Rome, the vigil office for the Assumption of Mary begins with Matins, as we shall see for Christmas. In the Ambrosian Office, the major solemnities of the place, such as Gervase and Protase, Nazarius and Celsus, Nabor and Felix, Simplicianus and Dionysius and other local saints also have a proper Mass office on the vigils. Other nations are also accustomed to do this for their patrons and similarly important saints, such as for Saint Lambert in Liège and Saint Martin in Utrecht.
According to the pious and religious custom of the more solemn churches, whenever fasting is observed on the vigil, and the de tempore service is celebrated at the hours with a commemoration of the saints, if any occur, as we do in the fasting seasons of Advent and Lent. And they say this should be piously observed likewise on other vigils of saints for which Mass is said but on which there is no fasting obligation.
The vigil Mass is said in ferial fashion on account of the fast, and—according to Micrologus chapter 55—“this, too, is appropriately observed on all vigils, namely that if None is postponed until after Mass, it should be said of the future feast. In other words, once we have begun the feast after Mass, let us not introduce any dissonance into the office at None, since the holy Fathers especially strove to avoid such dissonance in their arrangement of the offices.” And if the vigil falls on a Sunday, the Sunday office, which is greater, is not changed on account of it, and the Mass of the vigil should be said as the Sunday morrow mass, or on Saturday, or at some other apposite time.
Now that we have spoken about vigils, by which feast days are “begun earlier,” we must speak about octaves, by which they are “ended later.” In the first place we must note that according to the more approved uses such as that of the Carthusians and others, and as written in many liturgical commentaries, octaves are of two sorts: major and minor. In major octaves the first and eighth day as well as the intervening Sunday have nine lessons, while the rest have three. The office of the first and eighth day are identical to that of the feast. But the Sunday Mass is celebrated within the office borrowed from the feast day, because both Vespers use the psalms of the feast day, and at Vigils the Invitatory, hymn, nine responsories are from the feast, and likewise Lauds and the rest are of the feast itself. Further, the three nocturns use the Sunday psalms under three or nine antiphons, depending on how many the feast has; six of the readings are taken from the history that is being read at that time, along with the Sunday homily and the Te Deum; at both Vespers and at Lauds there is a commemoration of the Sunday; at Prime the psalms Deus in nomine, Confitemini, etc. The Mass of the Sunday is celebrated with a commemoration of the feast. On private days, the Invitatory will be brief and in the ferial tone, while the hymn is of the feast, the nocturn of the feria, recited under one antiphon from the festal nocturns, whichever is most appropriate, the lessons from the current history, the responsories from the feast in proper order, omitting the first according to Roman custom, and, once they are used up, beginning again from the second. Neither Te Deum nor Gloria in excelsis should be said, as shown in proposition 13; at Lauds use five antiphons over the common psalms; at Vespers five antiphons from Lauds over the ferial psalms, unless the ferial antiphons are used. The rest of the office is of the feast.
In minor octaves, the solemnity is not mentioned on any day before the octave day, which is kept with three lessons like any simple feast of three lessons. Hence we read in Micrologus chapter 44:
According to Roman authority, we must not observe the octaves of any saints unless we have a certain tradition to that effect from the holy fathers. And during those octaves which we do celebrate, there should be no daily commemoration on the days intervening, because we have no authority for such a thing, except for the Virgin Mary and for St. Peter, both of whom we never cease to commemorate even in other times.
We find the same in the commentary called Gemma Ecclesiae:
“Saints’ feasts of nine lessons within major octaves and to be kept with a commemoration of the octave. Let other saints be only commemorated; but if they should have a proper mass, let it be sung with Ite missa est.”
Christmas, Epiphany, the Ascension, Trinity, Corpus Christi, the Assumption, the Nativity of Mary, and Peter and Paul have this sort of major octave. Andrew, Laurence, and Martin, and, according to the Carthusians, John the Baptist have minor octaves. And note that private days within major octaves are regarded as if they were within Eastertide. Hence on those days there are no prostrations, preces with Miserere, and other things of that sort which are omitted on Eastertide; even the Carthusians observe this. Based on this consideration, many Germans on these days say only three psalms with three antiphons at Matins, as they are accustomed to do in Eastertide, but this practice was castigated above in Proposition 10.
 CJC, Decr. Greg. II, 9, 2 – Frdbg. II, 271.
 Pagi, Breviarium Rom. Pont, p. 1? in Greg. XI, Nr. 32; cf. Benedict XIV De festis BMV, pars II, 138, and O. Raynaldus, Annales Ecclesiastici, ed. J. D. Mansi, t. 7, 297 (Lucca, 1752)
 CJC, Decr. Greg. III, 46, 2 – Frdbg. II, 650–51. See proposition 17 and ML 215, 810.
 CJC, Decr. Greg. V, 40, 14 – Frdbg. II, 915 and proposition 17.
 If he refers to the Gemma anime (which often appeared under the title Gemma ecclesiae), then this passage is either a later addition or belongs to a tradition of unedited Gemma manuscripts.