We first discussed St Agnes on Distaff Sunday. St Agnes was a martyr who, at 13 years old, refused to marry a pagan, and was martyred as a result, by being stabbed in the throat. She is well attested and in a list of martyrs dating to AD345. She is patroness of young women and of chastity.

Folklore held that a maid could dream of her future lover on St Agnes Eve, if she took certain precautions. John Keats use this tradition in his epic poem, which begins with a great description of winter.

The Eve of St. Agnes

By John Keats

St. Agnes’ Eve—Ah, bitter chill it was!
       The owl, for all his feathers, was a-cold;
       The hare limp’d trembling through the frozen grass,
       And silent was the flock in woolly fold:
       Numb were the Beadsman’s fingers, while he told
       His rosary, and while his frosted breath,
       Like pious incense from a censer old,
       Seem’d taking flight for heaven, without a death,
Past the sweet Virgin’s picture, while his prayer he saith.

Keats sets up the drama with a poetic description of the folklore:

They told her how, upon St. Agnes’ Eve,
       Young virgins might have visions of delight,
       And soft adorings from their loves receive
       Upon the honey’d middle of the night,
       If ceremonies due they did aright;
       As, supperless to bed they must retire,
       And couch supine their beauties, lily white;
       Nor look behind, nor sideways, but require
Of Heaven with upward eyes for all that they desire.

https://www.poetryfoundation.org/poems/44470/the-eve-of-st-agnes

In the poem the maid Madelaine goes to sleep to dream of her love Porphyro . He risks everything to visit the young girl, and watches her while she sleeps. She dreams of him and seeing him when she wakes she lets him in her bed thinking she is still dreaming. They escape and run away together.