Ase Y me rod this ender day: introduction
London, British Library, Harley MS 2253, f. 81va-b

The poem opens in the manner of secular love-poetry, with the speaker riding out in spring for pleasure, preoccupied with thoughts of his beloved; but it soon becomes apparent that the beloved is the Virgin Mary, and the poem modulates into a hymn celebrating her Five Joys. 

The Five Joys of our Lady were the Annunciation, the Nativity, the Resurrection, the Ascension, and the Assumption (when she was taken up bodily into heaven); devotions based on them were popular from the thirteenth century onwards (for an early but extended example, see the elaborate set of prayers in Part 1 of the early-thirteenth century guide for women recluses, Ancrene Wisse) and became a standard feature---often expanded to fifteen Joys---of later medieval Books of Hours.