It's always a bummer when Halloween falls on a week day, but that doesn't stop Hollywood from whooping it up the weekend before! Stars like Jay Z -- yes, JAY was out in costume, but he hid when we tried to photograph him -- Kim and Kourtney Kardashian killed it as Madonna and Michael Jackson, Usher as Rick James?, Kelly Rowland as some sort of PVC-wrapped alien?, A$AP Rocky as ... himself ... and more were all out partying this weekend.

Matthew Morrison hosted an A-list bash at Hollywood hot spot Poppy and TONS of celebs turned out.

But dressing up isn't all fun and games, you've gotta hit the right note. And as happens every year SOME celeb does something deemed inappropriate and this year it was Kim K. The reality star dressed as the late singer Aaliyah for one party and fans screamed "Hell no!" Nevermind that Scream Queens star Kiki Palmer dressed as Aaliyah in 2015 and didn't get any flack for it ... she's not Kim K. Kim's not black, obviously, and even though she's married to Kanye, she's not allowed to dress as an African-American for Halloween; it just doesn't fly. Add to it that Aaliyah is ... dead ... and that doesn't go over well. Hmmm ... next year try Pocahontas, Kim!