When? It all depends on you and your tastes.
Ex-DW and I went there twice towards the latter part of October and LOVED the crowds, or lack thereof. We also went once on Jan 1 of 20?? (I don't remember exactly), JUST to save money. (prices were about 30% less on flights and rooms on Jan 1) Another time, our stay was from 12/27/03-1/3/04.
Each time, we loved the Disney "experience" for different reasons. Too bad we didn't take that feeling home with us, or "EX-DW" may still be "DW".
Good luck in your choice.
Dave