The first 2 weeks of May are when we usually go (althgouh this year we swung 2 trips, one in April, the other July)
May is my favorite time - weather is great, crowds less (althgouh still avoid MK on the weekend), perfect swimming weather.
Grad night is usually the first weekend in May (not sura about this year), so they sometimes close 1 park early on Fri night for that - also, Pleaseure Island is cloed early one night - otherwise it's sheer perfection.