Depends on where you want to sit. A lot of the good seats will definately be sold out. The shows themselves however probably will not sell out until closer to halloween - or on nights that MK has Mickey's Not so Scary - Cirque tends to be busier on those nights. Generally Wednesdays aren't as busy though.
At the VERY least, I would try to plan the show late in your vacation, and buy the tickets the day you arrive. It's hard to guess what the attendence will be. If there was a specific section you wanted to sit in however, you may just want to order them in advance. If you don't care and just want best availible Cat 2 or 3 - then you should be fine getting them a couple days in advance.
Although they only have shows Tue - Sat, the box office is open every day.