The Disneyland Resort really goes all out for their Halloween celebrations. Both Disneyland Park and Disney California Adventure attractions get layovers, there's endless amounts of seasonal food and drink available, characters are out in their Halloween costumes, and the parks are decorated with festive pumpkins.
One big draw from 2019 that's coming back in 2020 is Oogie Boogie Bash - A Disney Halloween Party at Disney California Adventure. This separately ticketed event is fun for the whole family. If you're planning on going you're going to want to get your tickets early since almost every night sold out last year.
Halloween Time at Disneyland Resort runs from September through October 2020.