McGill notifies in several cycles. If you are strong, you will be accepted in early Dec—this happened for my daughter and 2 of her friends ( 1 Arts, 2 Science students). This was great for us because it took a huge amount of stress off application season and allowed her to just focus on her reach schools for the Jan deadlines.
However, you might also be waiting until round 4 or 5–her boyfriend got into the science program at the end of June, and so did her friend from NYC ( who by that time had put down a deposit at another school) last year. Same with another high school friend of hers ( English major) two years ago— late June acceptance. All of these kids are now at McGill and loving it so when you get in does not matter at all!
But I actually now recommend the apply in October Canadian strategy to all my friends. If it works out, you will have an acceptance before Xmas at a great school that does not commit you to anything.