The Magnificat antiphon for the Feast of Our Lady's Nativity, Nativitas tua Dei Genitrix Virgo, is a translation of the Apolytikion of the Feast in the Byzantine Liturgy (Ἡ γέννησίς σου Θεοτόκε, χαρὰν ἐμήνυσε πάσῃ τῇ οικουμένῃ, ἐκ σοῦ γὰρ ἀνέτειλεν ὁ Ἥλιος τῆς δικαιοσύνης, Χριστὸς ὁ Θεὸς ἡμῶν, καὶ λύσας τὴν κατάραν, ἔδωκε τὴν εὐλογίαν, καὶ καταργήσας τὸν θάνατον, ἐδωρήσατο ἡμῖν ζωὴν τὴν αἰώνιον.).
Its Gregorian chant melody is interesting to me because it resembles a very ornate psalm-tone (rather in the style of that of the Ambrosian Transitorium Te laudamus Domine omnipotens), which I transcribe as follows, breaking the chant into sense-lines to reveal the repeated motifs:
Note that the first and fourth lines are all but identical as regards the chant, while the second, third and fifth lines are the same from the climacus (five descending notes) onwards, and after the quarter bar are all but the same as the first and fourth lines, just as the beginnings of the first and fourth (but for the climacus) resemble that of the second and to a lesser extent the fifth, while at the same time the beginnings of the third line is quite different in melody.
In the Dominican Rite, this antiphon is (or was) used also for Our Lady's Visitation and Presentation, as well as for other Marian feasts, in each case changing the word Nativitas into, respectively, Visitatio, Præsentatio, and Solemnitas. Likewise, in the Monastic Breviary, the same is done for the feast of Our Lady's Maternity, changing Nativitas into Maternitas (and ending the chant at Christus Deus noster).
We had a lovely OF parish Mass in the evening of the Feast yesterday; perhaps next year our schola can learn this chant and sing it either at Mass or Benediction.