Another vote for may.
We were there this year for 3 weeks,its hot,enough to give a very good tan!,no rain and shorts and t's the whole time,the air con is alittle cool,but you get that any time of the year.
Crowds are minimal,and the longest wait time we saw was 10 minutes,some rides were walk on!!.
Memorial day is a different story,but we plan well and did non park things instead.
It was that good,that i wouldnt think about going at other times of the year,and were already planning may 2005 for 3 weeks in May!!
Jules