There's not a lot of trickery going on here: the poem's title is "The Eve of St. Agnes" and it's about—wait for it—stuff that goes down on the… Eve of St. Agnes. If anything, it could be more of an exercise to think about what it's not titled—for instance, Keats decided not to go with The Ballad of Porphyro and Madeline or Closets: Not Just for Storing Lutes.
Throughout the poem you see various views about the Eve of St. Agnes, from Madeline's total faith in its powers to Angela's equally firm belief that it's phony baloney. Entitling the poem "The Eve of St. Agnes" draws your attention to one of the central questions of poem: does the stuff in the poem happen because it's the hallowed Eve of St. Agnes, or is something else going on here?
Everybody in the poem seems to have their own opinion about the St. Agnes mojo: Angela thinks it's baloney; Porphyro's worried about Madeline being disappointed and incorporates the myth into his plan: and Madeline herself has utter faith in it. The eve itself bookends the events of the poem: you start things in the evening, and the poem's climax wraps up just as "St. Agnes' moon hath set" (324). What power, if any, does the fact that it's the Eve of St. Agnes have over the events of the poem?