The Regina Caeli is the shortest and most jubilant of the Marian antiphons, four lines that tell the Queen of heaven to rejoice, alleluia, because he whom she merited to bear, alleluia, has risen as he said, alleluia, and that close by asking her to pray for us to God, alleluia. It appears in liturgical manuscripts around the year 1200 without attribution, and its author is unknown. The gap was filled, as medieval gaps usually were, by legend.
The Golden Legend of Jacobus de Voragine tells that during a penitential procession against the plague in Rome, Gregory the Great heard angels singing the antiphon's first three lines above the city, and answered them aloud with the fourth, whereupon he saw an angel sheathing a sword atop Hadrian's mausoleum, the castle thereafter called Sant'Angelo. The story is impossible, Gregory having died six centuries before the antiphon is attested, but it preserves what the medieval church felt about the text, that it sounded like something angels would sing. The antiphon's structure is itself a small Easter catechism, the resurrection announced to the person to whom the incarnation was announced, so that the Easter joy is offered first to Mary, and through her petitions to everyone else.