Until this year, April 15th was the latest date I had seen snow on my lawn. This year I had 7 inches of new snow on May 1st. The last killing frost this year (thus far) was June 6/7. It's historically May 13th for the last killing frost.
Summer can be quite warm and dry. I've experienced a high of 103F during the day with an overnight low of 86F. After a few of those experiences, I spent the money to repair the air conditioner. I haven't needed it yet this year. At the current rate, we won't use it at all.
I grew up in Calgary. One year it snowed on the last day of school, which in Calgary is June 30. A couple of years later it snowed on the first day of school, which if I recall was August 29.
Summer vacation for Canadian schools is only two months -- July and August. You can see why.