Growing up in the United States, I was always taught that the Summer Solstice…which occurs on or about June 21st each year…marked the "official" start of summer.
Not so in Sweden, where conventional wisdom says that summer starts about three weeks earlier than that (on June 1st), presumably because the season is so short. In fact, the Summer Solstice is a formal holiday here called "Mid-Summer" because when you’re this far north of the equator, June 21 pretty much is the "middle" of the summer season.
But the main reason I know it’s summer in southern Sweden is because it’s been raining for three solid days and the forecast is calling for the cloudy, wet conditions to continue for another week.
Summer my @$$.