Alright, let me make everything crystal clear.
McGill officially has two semesters----fall and winter. Fall semester starts on the first working day after labor day, i.e. usually the first few days in the beginning of Sept. Winter semester starts ten days or so after Christmas. A typical example could be Jan 3rd.
Summer semester is optional. Students can choose to take courses any time, provided courses are offered in the duration of the time slot they are interested in.