I have been to Halloweekends for the last three years, usually at the end of September. This has been my experience as far as crowds.
Friday: Very low crowds but not everything is open.
Saturday: Moderate to Heavy crowds, less than Saturdays in August but still pretty busy. However I will say that what with the park open until Midnight, the crowds do die down and the last three to four hours of the evening are very pleasant in terms of crowds.
Sunday: Last year was my only experience with Sunday, and it was a very good experience. Crowds were very light and lines were pretty short all day. Note that the park is only open from noon to 8:00 PM. This year I am planning two Sundays and one Saturday during Halloweekends.
Hope This helps,
CWD