For Easter. The one Marian prayer that is nothing but joy — four lines, four alleluias — and for fifty days it takes the Angelus's place.
It tells her the news. That is the whole of it: the one you carried has risen, as he said. Every line ends in alleluia because the prayer cannot get to the end of a sentence without saying it.
It is the only one of the four that is not addressed to her out of need. Nothing is wrong. And the one request, pray for us to God, comes almost as an afterthought, once the news has been delivered.
The words are twelfth century. The story attached to them is older and cannot be true as told: that in the year 590, during a plague procession in Rome, Gregory the Great heard angels singing the first three lines above the mausoleum of Hadrian, and answered with the fourth — ora pro nobis Deum. The building has been called Castel Sant'Angelo ever since. The procession is real; this site has it in the apparitions index. The antiphon is six centuries younger than the story that explains it, and the story was told anyway, which tells you how people felt about the prayer.
Two things. It became the night antiphon for all of Eastertide, Easter Sunday to the Saturday before Trinity. And it became the Easter replacement for the Angelus — for fifty days, morning, noon and evening, this is said standing instead. Most people who know the Angelus have never been told that.
It is sung far more than said. Mozart set it three times before he was twenty-five.