Catholic Encyclopedia (1913)/Invitatorium
The Invitatorium, as the word implies, is the invitation addressed to the faithful to come and take part in the Divine Office. The psalm "Venite" has been used for this purpose from the earliest times. In the life of St. Porphyrius of Gaza we read that this saint, wishing the people to join in prayer, caused the "Venite exultemus Domino" to be sung, and the people replied "Alleluia" after each verse. In the Benedictine Office the "Venite exultemus Domino" is recited daily at the beginning of the nocturns in the night Office and is called the Invitatorium. It is never omitted, but the antiphons that follow each verse are changed according to whether it is a ferial or a saint's Office that is being recited. These antiphons are repeated twice before the psalm and once after the "Gloria Patri ". The Rule of St. Benedict calls this psalm the Invitatorium, while the Rule of the Master (Magister Anonymus, a Frankish author of the seventh century) calls it the Responsorium hortationis. The Mozarabic Liturgy makes use of an expressive word: sonus, as if to signify the bell that calls to the church. The most ancient Roman Liturgy we know of did not contain an Invitatorium; for it is omitted in the primitive liturgy, which is represented in our days by that of the last three days of Holy Week. If we find it in the Office of the Dead, it is because it was introduced at a later period. The Council of Aachen (816) mentions the invitatory psalm "Venite" and forbids its use in the Office of the Dead. This same canon, in speaking of the manner of reciting the Invitatorium, employs the very words of the Rule of St. Benedict, which shows clearly that the use of this psalm was closely connected with the monastic Office.
The Invitatorium was purposely said slowly, like the preceding psalm: "Domine quid multiplicati sunt". This was to enable the monks who were coming to the vigil to arrive in time for the beginning of the Office. Indeed, it really seems that these two preliminary psalms (Ps. iii and xciv) were the prayers said privately by the monks while rising and coming to choir: "Ego dormivi et soporatus sum et exsurrexi." It is possible that in the course of time the custom was introduced of reciting them aloud in choir, while awaiting the arrival of those who were late, and thus, after a while, they were inserted in the Office itself. In effect, the psalm "Venite" would seem to be addressed to those who were to come to the vigil rather than to those who were already there. At Rome, on the feast of the Epiphany, there was no Invitatorium. The psalmody began, and still begins, with the psalms of the first nocturn and their antiphons. "Hodie non cantamus Invitatorium sed absolute incipimus" (To-day we chant no Invitatory but begin without it) is an instruction in a rubric of the Vatican antiphonary. The psalm "Venite" was recited with its own antiphon in its proper place, that is to say, the last of the psalms of the second nocturn. Later this psalm became the first psalm of the third nocturn, and the antiphon was repeated just as when it was used at the Invitatorium. Amalarius and Durandus of Mende try as usual to explain it mystically, but the most probable explanation is that the Invitatorium was suppressed because the psalm was recited later and they did not wish to recite it twice in the same Office.
The Benedictine Breviary, which had hymns for its third nocturn, had not the same reason for excluding it and so retained it on the feast of the Epiphany. We see, nevertheless, that, before the ninth century, the Roman Liturgy had not the Invitatorium, at least not as regularly as the Benedictine Liturgy. It is likely that it was first introduced out of imitation of the monastic practice, on those days alone on which the people assisted at the vigil, when the Invitatorium would thus be addressed to some one. The "Ordines Romani" inform us that, on great festivals, two nocturnal offices were celebrated: one, without the Invitatorium, was recited by the priests of the papal chapel in their chapel; the other with the Invitatorium, at which the people assisted. Amalarius tells us that in his time only the Office for the vigil of Sunday had the Invitatorium, the ferial Office had not, because the people did not assist at it. On the feast of the Commemoration of the Dead the Invitatorium was recited, because the faithful came that day to pray for the deceased, but this brings us to a much later date. Most likely the origin of the Invitatorium is to be found in the call by which the monks were awakened: "Venite adoremus Dominum", which soon became the anthem or the refrain of the psalm "Venite exultemus Domino" which this prayer naturally recalled. Amalarius calls our attention to a peculiar fact. On week-days the Invitatorium was recited without the insertion of the antiphons: "Invitatorium diebus festivis hebdomadibus sine modulatione Antiphone solet dici." The version of the psalm "Venite exultemus" used in the Breviary is that of the ancient Roman psalter, which differs in some passages from the Vulgate.