At length burst in the argent revelry, With plume, tiara, and all rich array, Numerous as shadows haunting fairily The brain, new stuff'd, in youth, with triumphs gay Of old romance. These let us wish away, And turn, sole-thoughted, to one Lady there, Whose heart has brooded, all that wintry day, On love, and wing'd St. Agnes' saintly care, As she had heard old dames full many times declare.
Here come the guests, who, frankly, sound like our kind of crowd: they're decked out in feathers and tiaras, which sounds not unlike Vegas.
Pay attention, though, to the way in which this language is more evocative than truly descriptive. For instance, what the heck does "argent revelry" mean? "Argent" means "silvery," and a "revelry" is a party, but a "silver party" doesn't form a real, concrete image. Instead, you just get the feel of what Keats is getting at (bright, luxurious, noisy festivities).
Things get even murkier when the partiers are compared in a simile to "shadows haunting" like fairies might do.
These people sound kind of imaginary. They're not just "shadows haunting" the brain, though, but those haunting a brain that's "stuff'd with old romance," which is a figurative way of describing how incurably romantic young minds can be, before the harsh realities of life set in.
Whoosh! In comes the narrator (he and you make up the "us" of line 41), looking down on the scene. Then suddenly he (or she, it's not clear here) sweeps his hand and wishes all this stuff away.
That's because you guys have got something better to concentrate on: a lady. And unlike the rest of the revelers, she feels very real. She's been spending the entire frigid day thinking (unlike the other party-guests, she's actually connected to the physical, freezing outside world). She seems sad, contemplating (brooding) about love and the stories she's been told about St. Agnes by old ladies.
So, the million dollar question is: just who is this St. Agnes chick? Well, she was a 13-year-old Christian martyr who died in the fourth century and was named the patron saint of virgins by the Catholic church. Good to know.