We went to mnsshp 10/25/03 (Sunday before holloween) and the crowds were not to bad all day at the MK. Less after the party started 6-7pm.. We walked right on most all rides. We road buzz 6 times as fast as we could get off and walk back around. Space Mountain was the same way. The only line was for Splash Mountain & it only took 10 minutes. Thats was because it was parade time. After the Parade was over most of the people with younger kids left (9:30-10pm). At closing time there wasn't even a line for the boat back to FW. Which surprized us because there was so many people going to mnsshp from the FW campground that Disney sent the big Ferry Boat from the TTC to Fw to take all the people waiting to go to the MK for the party.
I heard the mnsshp was sold out for 10/31/03 from our neighbor at FW he said "it was a VERY tough night at the MK". He waited for everything.
I would say not to do mnsshp on the 31 but any other night should be fine, with a date farther from the 31 the better.