Every year, during September and October, Disneyland (and World), decorates for Halloween. There's even an after-hours party. Tickets go on sale later in the year, but get yours ASAP! They'll sale out quickly!
We attended in October 2015, and we had a blast! It was my first time going (my wife had already gone earlier in her life...). We dressed up as Robin and Maid Marian from the animated version, Robin Hood. Our costumes didn't include any masks, which is a good thing because adults are not allowed to wear masks of any kind.
There isn't much to it, but it's a great experience; especially for little ones. It grants you an opportunity to visit the park after-hours and to ride some rides that you may not be able to do during regular hours. For instance, we rode Peter Pan's Flight. The line is usually extra long (I think there should be a fast pass for it...), but with less people in the park, I think it was shorter. I have to say, Peter Pan is one of my favorite characters and I'm nervous to meet him when we go in March 2017. (Stay tuned, as I'm working on a secret costume for a local comic convention.)
We didn't focus much on meeting the characters (since it was just the two of us) and especially since the lines were crazy long. On top of that, it rained a little bit while there, so they had to move some of the characters to dry spots. They even had to move some of the treat stations too. What's a treat station you ask? Well, it's exacatly that. It's a trail that winds throughout the park and you get candy as you travel it. It's quite a lot of candy, and I doubt we ever finished it all...
What's also cool is that The Haunted Mansion is also "remodeled" to be The Nightmare Before Christmas. Since this was my first time attending Disneyland during a holiday, I had no experience what-so-ever. It was great, though! I'm a big fan of the movie, so it was awesome to see the Mansion decked out! I'd gladly go again!