I've actually been all of the months you are questioning....uncanny! ....I frequent in December and it is my go to month. I have strayed from my precious Dec only a few times, once in Jan (1st/2nd week), once in March (2nd week), once in May (3rd week) once in Sept (so far back that I can't remember much detail) ...and I'm about to stray this upcoming September again. December is incredible...our stamina is at its best, we love the decorations/atmosphere, and if you visit before the Xmas crowds, even better. The catch: If your intentions are to have swimming as a heavy part of the itinerary, whether at a resort or a park, you never know what will be dealt. We had vacations when it was actually TOO cold the entire trip...and kiddos arent weanies to the cold. However, seeing your breath in the air and having a thin coat of frost on the car windows...uh...
....yeah, sorry kids, no swimming.
The other big catch: rides seem to be less available. I'm not talking the major shutdowns that we get advance notice of, I'm talking little ones here and there that Disney sneaks in without an official "announcement"....you just show up to the park and without a warning, a headliner is closed for 2 or 3 days for maintenence.
Not a big deal and perhaps not long enough to release an official announcement of it's temporary closure...BUT...what if that was the ONLY 2 days you planned to be at that park your entire life??
That's all I'm saying. LOL..they'd NEVER sneak these kinda ninja moves in during Spring/Summer...never!!
You asked of March...completely fine...weather was warm but not horrible..and I went early enough to beat Spring Break crowds (2nd week), no crowd or closure issues.
May: might be my least favorite...the horrible heat already set in for the summer, I was not pleased. Kids were already on summer break so crowd levels were higher than I was accustomed to. And this may not affect your trip.....but my kids were TERRIBLE, I mean...whoa!! However, the parks were open very loooooong hours so park time was most ample.
You asked of Sept..my memory is vague, but I do recall uncomfortable heat and tons of early MK closings, I'm sure due to
MNSSHP...i DO get a refresher in 45 days though...can't wait!!