Honestly, if you can only go on Friday, go on Friday. We were stuck going ON Halloween this past year and I was very worried about the crowd levels, but it actually wasn't that bad. Remember, they limit the number of people who can get in the pack each night, so even when it's sold out it can't be THAT crowded.
We went to the Christmas Party on a Friday night in 2004, and thought it wasn't crowded at all.