July 1 is indeed a very important date for most people living in Quebec. It's a date when a lot of people move, and people in Quebec move a lot; apparently Montreal is a world champion in that regard.
We receive address changes in the weeks that follow and, by mid-September, we have received most of them. By the third week of September, we are still receiving quite a lot of them, but by the middle of that month, we have received most of the address changes, so that if an election were to be held in mid-September or, preferably, late September, most of those changes would already have been reflected in the list.
Of course, we have an agreement with Elections Canada whereby we can forward on request, the actual day of a federal election, the updated electoral list.