I've had luck in early May, early September, and I used to have luck with late February/early March (but weather where I live is always an issue and prices keep going up during that time frame). If you have freedom to go whatever time of year you want, try and think of something you've always wanted to do and work backwards from there. If you want to do Flower and Garden, Food and Wine, Halloween, Christmas, or Easter you can look at crowd calendars and find a decent week during those times to go. Also check the refurbishment calendar, if you're going solo you may want to go on your favorite ride over and over again so make sure it's up and running.
Lastly, I'd advise avoiding any holiday normal people would have off if you can. Summer/school break/weekends are all going to be busier. If you can go during a non-break and have most of your days fall on weekdays, you'll be golden.
