Hey there Alicia,
Welcome to planDisney and thank you so much for this question! My son and I attended
Oogie Boogie Bash - A Disney Halloween Party last fall for the first time and it was incredible! We had such a fun time seeing all of our favorite villains (Sid and Dr. Doom were the highlights for me last year) and loved all of the treat trails.
This is a separately ticketed event, so we had purchased
standard theme park tickets with the
park hopper option and spent the day leading up to the event between both
Disneyland Park and
Disney California Adventure Park. Then, around 4:00 PM that afternoon, we went back over to Disney California Adventure Park and settled in until the event started at 6:00 PM.
If you purchase tickets to Oogie Boogie Bash - A Disney Halloween Party this year, make sure to read the ticket details because in previous years they have allowed you to enter Disney California Adventure Park a bit earlier than the event start time. If you're only interested in attending the event, then it might be worth just purchasing the event tickets and skipping the standard park tickets for that day. I've heard other people who travel to the parks and attend the special event on the evening they arrive so they don't need to purchase park tickets for that day, which can be a great way to save a bit of money.
Either way you go, you're going to have a wonderful time! Thank you again for your question, Alicia, and have a wonderful trip.
Have a Magical Day,
Greg