Met Masquerade Galas

 As the New Middle Ages is an inherent anachronism, it is filled with "old timey" things that were on the cusp of the Early Modern Period, but are common in Neo-Medievalism, because they sort of bridge that gap between old and new, with people not fully grasping which is either. Something akin to a cultural mobius strip.

Masquerades are quite common in Medieval America, at least among the wealthy, urban populations, which is admittedly not a lot. A catch-all theme is spooky, or mythical figures. But it also very common, even for Halloween, to have very specific themes, which evolved from the Met Galas, which happened in September, and seemed to grow in popularity--they were practically a private affair pre-00's, and in many ways this has recursively become that way again in the 00's. Much in the way most Medieval American states sort of claim to be the "true" successor the United States, as the rich and famous could no longer travel to new York City, and in fact, New York City itself was hyperdecimated, cities across America--Cincinnati, New Orleans, Sacramento and Seattle, all declared themselves to be the homes to the Official New Meta Gala.

The Gala had a  relative proximity to Halloween, and as festivals in the New Middle Ages tried to affix themselves less to exact dates, than floating periods that worked to be whatever times was most doable, once or twice the two celebrations crossed paths and it didn't take quite long for a major conflation, and a great deal of Halloween Parties and Met Galas are the same thing.

The Met Masquerade, as mentioned above, definitely try conform to an overall theme, especially as embracing outright macabre costumes tend to fall in and out of style. Sometimes they're odes to Classical Mythology (Figures that have been around for thousands of years will surely stick in collective memory for another thousand.) Sometimes they're animal themes. Sometimes they're takes on other nations, particularly ones further away. Those takes are not particularly accurate, as communication is low and probably so is cultural sensitivity.

Costume prizes are handed out of course. Unlike knights, there's probably not an extensive enough masquerade circuit. As it is, the nature of these costumes require a good deal of resources to make, so prizes just barely make the rich get richer, but many times being the modern equivalent to a cosplay champion is a real money sink.