The Lenten-season antiphon of Our Lady — 'Hail, Queen of Heaven' — found in manuscripts from the twelfth century, sung at Compline from Candlemas until Holy Thursday.
Hail, O Queen of heaven, enthroned! Hail, by angels Mistress owned! Root of Jesse, Gate of morn, whence the world's true Light was born: Glorious Virgin, joy to thee, loveliest whom in heaven they see: Fairest thou where all are fair, plead with Christ our sins to spare.
The dated sources the library holds, oldest first — in their own words.
“It is found in the St. Alban's Book of the twelfth century; in a Munich manuscript thought by Daniel to be of the thirteenth; in a Sarum Breviary of the fourteenth; and in York and Roman Breviaries of the fifteenth.”
One of the four seasonal antiphons of Our Lady sung at Compline, the Ave Regina Caelorum is assigned from Compline of Candlemas (2 February) until Holy Thursday, when the Church keeps silence before the Passion.
With the Alma Redemptoris Mater (Advent to Candlemas), the Ave Regina Caelorum (Candlemas to Holy Thursday), the Regina Caeli (Eastertide), and the Salve Regina (the rest of the year), the Church closes each day at Compline with a seasonal anthem to Our Lady.