So, it's Advent. Advent Ferias are 3rd class feasts, but 3rd class saints displace them to commemoration status, so the antiphons are "Extra Tempus Paschale", not for Advent in that case.
Today is the Feast of the Immaculate Conception. Does one sing the Benedicamus Domino for 1st class feasts (at Vespers) or for feasts of the Blessed Virgin Mary?
In the Antiphonale romanum (1949), which appears to be the last edition for Roman Office, the rubric directs one to use, in major Marian feasts, the Benedicamus tone prescribed for other solemnities (in festis solemnibus). No matter if it's Advent or not.
Chris, it seems to me that the assignment of melodies to the Bendicamus is probably not absolute, any more than the Kyriale is for Masses. But I’d say that for the most solemn Marian day in the calendar, use the first class one.
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