We went on Halloween night on our trip last October. We were there the last week in October and I could have gone on other nights but I figure if I'm there for Halloween, I might as well do the party on Halloween night.
I was a bit worried about the crowds but it worked out fine. We made sure we were in the park before it opened by eating at LTT. Then we did our free photo right outside the restaurant and were first in line! Once the park was open, we headed to Haunted Mansion and there was no wait to ride it.
Then we started to see the crowds. We were mostly interested in Trick or Treating since it was Halloween and although the lines looked long, they went really fast. We worked our way around the park Trick or Treating and the only ride we did go on was Goofy's Barnstormer.
Then at 9:00p we headed back to Fantasyland to do some rides but our DD got sick (that virus that's been going around WDW/
DCL). She was so bummed. We ended up missing the fireworks in search of a change of clothes but I've heard they are amazing (like
MVMCP's). She begged us not to leave so she could see the second parade but she fell asleep. We couldn't wake her. So we stayed and saw the parade and taped it for her. It was very good, especially the Headless Horseman!!
All in all it was crowded even during the second parade but it was great trick or treating there on Halloween. Our DD wants to try again when she's NOT sick so we're going again for Halloween in 2004 (X-mas this year).
So basically, if you are going mostly for the rides, it's crowded. If you are going for the atmosphere and trick or treating it's GREAT!
Have fun!