For Advent, and the dark end of the year. It is the prayer of someone who has fallen and is trying to get up — it says that in so many words — and it asks her for help with the getting up, not with the fall.
It opens with two of her oldest names, gate of heaven and star of the sea, and then says the plainest thing on the page: help a people who have fallen and are trying to rise. Not people who have fallen. People who are trying to get up. The prayer assumes you are already trying.
Then it turns to the strangest fact about her, and lets it stay strange — you bore your own Maker, while nature wondered. And it ends where Advent ends, with Gabriel's greeting and one request: have mercy on sinners.
The eleventh century. It has been ascribed to Hermann of Reichenau for a very long time — the same monk the Salve Regina is credited to, born in 1013, unable to walk, carried from room to room, and one of the best minds of his century. The ascription is old and it cannot be proved; the manuscripts do not settle it, and honest editions say so. What is not in doubt is that it was being sung by the eleven hundreds and has never stopped.
The clearest thing that happened to it is a seven-year-old in a poem. In Chaucer's Prioress's Tale, written about 1390, a schoolboy hears the older boys singing the Alma Redemptoris and learns it by heart without knowing what the Latin means — I lerne song, I kan but smal grammeere. That is how it actually spread: not read, but overheard and kept. By Chaucer's time an English child could be expected to know it.
It is still the night antiphon for Advent and Christmas, from the first Sunday of Advent until the second of February. If you are up late in December, this is what the Church is singing.