This is a rubric for the hymn Ave maris stella from a French 13th century source. Someone helped me and gave me: 'A song for the Blessed Virgin when her office is celebrated on Saturdays in the monastery.' I'm looking for a literal translation. Is this close? Thank you.
According to https://cantus.uwaterloo.ca/chant/675638, the correct reading of this line in a Dominican hymnary is indeed Cantus de beata virgine quando in sabbatis de ea agitur officium in conventu, which indeed translates as "A hymn¹ of² the Virgin Mary when her office is celebrated on Saturdays in the convent³" [Footnotes: ¹ or "song" or "chant", but "hymn" would be the usual liturgical term; ² or "about"; ³ or "monastery", but this manuscript was for the use of Dominican nuns, so "convent" would be the more usual word].