One would think that Sat. would be the most crowded day, and so the most crowded EE, but I'm not so sure. Sat. is more crowded because there will be more 1 and 2 day visitors (who don't have EE). Also, people do have to get up much earlier on Sat. for EE since the park opens a bit later during the week during the off-season. I've been to close to a dozen EEs over the past 2 years, and the least crowded one was on a Sat, although in that particular case, I'm not sure why. Perhaps it was a fluke. Nevertheless, I wouldn't over worry about it; if you don't mind getting up really early and want to spend some fun time with you family, go on Sat.
(But if you don't want to get up by 6AM for a 7AM EE, and you want the best shot at the smallest crowds for EE, JenDisneylandlver is probably right; Monday probably has the smallest EE crowds on a whole since Sundays are often a travel day and hotels seem least likely to book up on Sun.)