"St. Agnes! Ah! it is St. Agnes' Eve— Yet men will murder upon holy days: Thou must hold water in a witch's sieve, And be liege-lord of all the Elves and Fays, To venture so: it fills me with amaze To see thee, Porphyro!—St. Agnes' Eve! God's help! my lady fair the conjuror plays This very night: good angels her deceive! But let me laugh awhile, I've mickle time to grieve."
Angela thinks that Porphyro's mentioning the holiday because it'll make it less likely that Madeline's family will kill him. She's quick to disabuse him: "Nope—they're just as happy to murder you today as any other day. And that would be very, very happy, for the record."
Once again, we have an invocation of fairies: Angela can't even believe Porphyro got into the castle unscathed, and says that he must be some sort of elf-king, capable of witchcraft, to have pulled it off.
Angela thinks that this whole plan of Madeline's to see her future husband is silly, and she laughs about it.
It's kind of funny that she calls Madeline a "conjurer," though—so far, the St. Agnes' Eve rituals have been described so that you have this idea of Madeline making herself totally vulnerable so that she can be receptive enough to get a vision from heaven. Here, though, Angela makes it sound like Madeline is herself controlling the ritual, as if she's a magician pulling a vision out of her hat.
Angela thinks it's all baloney: Madeline's putting her trust in a worthy saint ("good angels"), but this whole presto-magic-vision thing ain't gonna happen, if you ask her.