I don't think the beginning of summer occurs on the day of the summer solstice. Summer, is not an astronomical event, it pertains mostly to how warm the weather is. If you want to go by astronomy then the middle of summer should be on the summer solstice. But the thing is the highest temperatures lag the sun angle by some amount. This is mostly because it takes time for things to heat up, and to lose their heat. If it didn't, then the hottest day would be the summer solstice. Around these parts the highest temperatures lag the sun angle by about 5 weeks, so that the end of July is the warmest part of the year. So, that's midsummer. There are 3 months per season, so a month and a half on each side of the warmest days. That's 45 days. If July 25th is the middle of summer, that means that June 10th is the start of summer, and Sept 8th is the end of it.
So, the start of summer is about 12 days before the solstice, and the end of it is about the same amount before the equinox. There's also the sun angle to consider though, because the sun is stronger in June than in September, and what makes you feel warm is not just the ambient temperature but also the direct heating you receive from solar radiation. This would tilt the balance so that summer should begin and end even earlier. It is for this reason that I think memorial day and labor day are perfectly suited for marking the beginning and end of summer.
It used to be that the Summer Solstice was considered the middle of Summer. Take Shakespeare's play, a Midsummer Night's Dream, which I believe takes place around the summer solstice. Why is the first day of summer considered to be the summer solstice then? I don't know, but maybe it has to do with the fact that it's a distinct event that's closely linked to the warmest part of the year. And when did the first day of Summer become the Summer Solstice? I don't know either, but I can't help but think it has something to do with the industrial revolution.
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