system
A chat on subreddit /r/NoStupidQuestions
human
That's it.  When school was becoming widespread, kids helping during summers was not really negotiable.

Now, continuing the break seems to give more benefits than detriments, and summer seems the best time for it.
gpt
I've heard otherwise. I'm no farmer, but I understand that a lot of major crops are harvested at the end of the growing season/summer. The peak time for harvesting is when kids are just starting their school year.

The understanding that I have is that the traditional breaks predate universal education where people poor enough to have their kids work fields were poor enough to not get much if any formal education.

The break for the hottest part of the year was for comfort. In a world before air conditioning, convenient showers, deodorant, or even electric fans,  would you want to be in a room full of teenagers?