"The Eve of St. Agnes" begins in the countryside around a castle, deep in a merciless winter. The festival the poem is named after—the day sacred to St. Agnes, the patron saint of young women—falls at the end of January, and in this poem's world, it seems to have been a particularly harsh month. Night has fallen, and everything is "silent" and "bitter chill": the whole world is anxiously frozen. Even the "owl" and the "hare" are shivering as they make their way through the frosty landscape.
From these pitiful animals, the poem turns to another sorry figure: the Beadsman, a religious servant hired to pray ceaselessly for the souls of a noble family (who presumably have better things to do than praying for their souls themselves). He's called a "Beadsman" because his job is to say the rosary over and over—a kind of prayer counted out on beads. His arrival lets readers know that this poem is set in a distant English past (the ancient Catholic tradition of his flavor of constant prayer died out after the Reformation in the 15th century).
The poem's stanza form also hints that this poem is going to take place in a long-ago world. The eight lines of iambic pentameter (that is, lines of five iambs, metrical feet that go da-DUM) capped with one line of iambic hexameter (six iambs) identifies this as a Spenserian stanza: a form invented and popularized by the Renaissance poet Edmund Spenser in his great verse romance The Faerie Queene. (See the Form, Meter, and Rhyme Scheme sections for more on that.) That story, with its knights and fair ladies, is going to be a big influence on this one: "The Eve of St. Agnes" is a dyed-in-the-wool romance. That doesn't mean it's a love story—though it will also be that!—but a legendary tale of medieval chivalry and magic.
Right from the beginning, the poem immerses its reader in this romantic world with rich sensory imagery: the "silen[ce]" of the sheep, the "trembling" of the hare, and the "frosted breath" of the poor old Beadsman as he prays in a frigid chapel. The first of the poem's many vivid similes also appears:
[...] 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.
Here, the Beadsman's breath looks like the incense one might burn during a Catholic Mass—a pretty fitting thing for a holy man's breath to look like. But it also looks like a spirit ascending to heaven. This complex simile prepares readers for another of the poem's themes: dreams and the imagination. This is a world in which everything is like a dream, layered with mysterious meanings.