For the Pre-MBA Program, the start date is as mentioned by achakraverty, August 4. However, the schedule isn't out yet.
I'm just going to correct achakraverty a little here. Orientation is August 26-28. It is the last Tuesday, Wednesday and Thursday of August, and it is mandatory. Orientation camp is August 29-31, the Friday to Sunday after Orientation. While every Rotman student and alum I spoke to say I must attend, it is not mandatory. However, it is organized by students, and run by students, and no staff is on camp.
First day of classes is after Labour Day, September 2. Exam week for the first term is the week of September 29. Sigh...