Are you staying onsite or offiste at Disney? If you're staying onsite at WDW, I'd say do it at the end of the trip. If you're staying offsite, I agree with another poster that it may help break up the week a bit if you go mid-week. Also, keep in mind that US/IOA, like WDW, is busier as the weekend approaches. Our favorite days to do any theme parks are Tuesday, Wednesday and Thursday. The weekend crowds usually start arriving on Fridays and sometimes stay until Mondays. Good luck planning your trip!