Dork Diaries - Party Time by Rachel Renee Russell

Nikki is finally starting to adjust to life at her new school and things are looking up. She's made some real friends and her major crush, Brandon, even asks her to be his lab partner in science, Hello! Seriously awesome development! But when Nikki overhears mean girl Mackenzie bragging that Brandon's going to take her to the Halloween dance, a bummed Nikki agrees to spend Halloween at a kids' party for her little sister Brianna. It's only after she makes the commitment that she finds out Mackenzie was lying and that her dream of going to the party with Brandon could be a reality. Now she's got two parties to juggle, what's a girl to do?