We've tried several weeks during the Oct.-Nov. time frame, then tried September this year. Hands down, it was the best time we've ever had. We did TL on a Monday and litterally got cramps from running up the stairs of the slides so many times, due to no lines. Wednesday in MK, we spent 40 minutes walking back and forth between Splash Mountain and Big Thunder, walked on both each time. Epcot was light, MGM almost abandoned (all week days). The biggest surprise was going to MGM on Saturday and finding it crowded enough to make things feel like Disney, yet we had five minute waits for R&RC and ToT.
But a warning.....we'd never seen Fantasy in the Sky because we just hate crowds that much. We left MGM Saturday night and went over to MK around 6. The place looked like Spring Break. I'm GLAD I saw the fireworks, but have no illusions that what you see elsewhere during September will be replicated on Saturday at MK.
That aside, we've vowed we'll never go anytime but September again. Then again, my job wants me to go to a seminar at the Wydham in June. Hmmmmmm.................