When to go:
5th - 7th October - Off peak
Park should be quiet but major rides run on low capacity, should easily get everything done if you arrive for opening, but lines that look short can still be 20+ minutes.
8th October - Halloween event preview night, only open to annual pass holders.
9th October to 11th October - Halloween event
Park is open until 10pm so you can have scare mazes or a couple of rides in the dark if you're interested.
Friday 9th will probably be best as less people will be there in the morning and you can get most rides out of the way before the evening. Hopefully rides will have more trains on too!
Once the crowds arrive expect queues of 60+ minutes for major rides and all mazes and not being able to get everything done.
12th October to 15th October - Off peak
As above
Enjoy your trip!
P.S. Slammer closes at some points during the day. If you see it running, take the opportunity to hop on before you miss it.