Guess Who's Katy Perry's Grammy Date? [HINT: Not John Mayer]

Remember, Katy Perry Kissed a Girl and Liked It ...

Not that her date for Sunday's Grammy awards will head in that direction, but Katy Perry is taking a girl as her "plus one."

Even though Katy Perry and her boyfriend of several months, John Mayer, are apparently still going strong (they've been house hunting together), she has decided not to bring him to Sunday's big music awards show.

Who will be on Perry's arm instead? Allison Williams, co-star of HBO's critical darling "Girls." (Who also just happens to be NBC News anchor, Brian Williams', daughter.)

How'd that come about? Williams' boyfriend, Ricky Van Deen is super tight with Mayer and she and Katy have gotten close. As Williams explained to Anderson Cooper, "I am going to the Grammys. Katy Perry is my date. She is the hottest date. She's a friend of mine and she's nominated, as she should be, and I can't wait to go. She's awesome. She's the best."

READ: Justin Timberlake Will Perform at the Grammys - First Time in Four Years!

True enough and Perry always makes a splash at the Grammys -- especially on the red carpet, though this year she may just have to err on the side of subdued. Why? The brass behind the awards show, which is celebrating its 55th year, are looking to class things up a tad. Producers have sent out very detailed marching orders on the evening's suitable dress code. E!Online reports that the big shots will not tolerate butt crack, sideboob, sheer clothing and a whole host of other improprieties.

"Please be sure that buttocks and female breasts are adequately covered. Thong type costumes are problematic. Please avoid exposing bare fleshy under curves of the buttocks and buttock crack. Bare sides or under curvature of the breasts is also problematic. Please avoid sheer see-through clothing that could possibly expose female breast nipples. Please be sure the genital region is adequately covered so that there is no visible 'puffy' bare skin exposure."