I'd go earlier in May, before the colleges let out. I've been there before during that time of year and we counted more ducks than people one morning in Adventureland!
We were just married on May 10th and the parks were pretty quiet. Space Mt had a 30 minute wait in the middle of the day. The only place we saw crazy crowds was Downtown Disney.
I would say right after the Easter/Spring holidays is the perfect time and that would be the beginning of May. Not the very first week though because the extended Easter vacationers are still there but probably after the 5th of May.
I love the beginning of May because the parks are so quiet, I wish I could go more often but I'm a teacher and can't get the time off
weekends would most likely be busier for weddings, although being a destination wedding place some brides don't care about weekend/day...Like me - I'm doing a Friday! I went last year to WDW the second week of May and it was pretty average in terms of crowds, at the end of our vacation (may 10th-18th) the crowds were picking up. I did a lot of research on the crowds in WDW before we went and it seemed like the best option before the summer and after the easter/spring break rush