I used a recent Trivia Tuesday to provide an answer to this question! Happy to answer any follow-up questions you might have!
So, let's take a moment to talk about why it is American schools take a break for 10-14 weeks every year, and most of that break is held during the summer. Odds are good if we polled 1000 Americans why they think schools are off in the summer, a fairly large number of them would say it's so children could work on the farm. The idea that the American school year is based on an "agrarian calendars" generally started in the 1950s as school administrators pushed for larger budgets to extend the school year and school day. The reference is meant to convince the speaker schools need to change to break away from outdated practices. From a 1966 article covering a guest speaker at a district budget meeting:
In his talk. Dr. Thomas said for the past 125 years the school has been following the agrarian calendar to allow boys to work on farms, limiting the school year to 180 days and asked, "Can we afford part-time education where so much is new such as in history, math and science?"
To be sure, there were districts in the United States, especially in the South, where summer crops were the norm and young people of all genders were needed to help. This, though, is unrelated to summer vacation which started on the East Coast of the United States, primarily in New York City in the late 1800s.
To recap: Public schools for the common good increasingly became the norm in the decades leading up to the Civil War. States established compulsory education laws, tax structures, teacher training schools, and going to school was something children began to do. However, in most cases, going to school meant attending one rf two sessions a year: either the Winter or Summer session. Both were typically 6-8 weeks long and the Winter one skewed older and often had a male teacher and the Summer session skewed younger and usually had a woman teacher (typically paid 1/2 to 1/3 as much as the male teacher.) This structure spread across the country and slowly, the winter teacher became the same as the summer teacher who sometimes offered a Fall term and more and more parents sent their children.
In the case of New York City schools, the buildings, taxes, and teachers were mostly in place by the end of the Civil War (especially for white Protestant children. Catholic parents were becoming increasingly uncomfortable with the use of religious texts in schools but that's a different post.) Part of the emerging school schedule was to keep schools closed when it was clear parents weren't going to send their children. This included major Christian holidays and some NYC-American-centric ones such as the 4th of July, Thanksgiving, Brooklyn-Queens Day, etc. Additionally, health advocates who worked with schools pushed for schools to close when keeping them open became a health risk. In places where cholera outbreaks were common, schools would close when officials feared an outbreak was imminent or suspected one was happening.
In the late 1800s, two schools of thought converged to result in the closing of schools from the end of June to after Labor Day. The first was the idea that people got sick from bad air. So, there was a massive push to rehab schools, get students fresh air, access to running water, and bathrooms. This was impossible to do while children were physically in the building so a large-scale closing of schools made sense. (Some city leaders pushed for rolling closings. But even in the late 1800s, adults knew that if children knew kids in other parts of the city were on vacation, getting them to attend would be harder.) This idea that "bad air" contributed to bad health also informed the travel habits of the city's wealthy. It became routine for families who could afford to leave NYC-proper to head to the towns and communities north and east of the city to avoid the "bad" air of NYC in August. (NYC during a heatwave is a deeply uncomfortable experience, even in the modern era.) As train travel became more affordable, middle-class families joined the exodus, and this included teachers. It didn't take long for those funding schools to realized it was expensive to keep them open if the classrooms weren't filled or students were home idle because their teacher was on vacation. Again, there was some pushback. Mostly related to the idea of having roving bands of children lose on the summer streets.
This pushback, though, connected to the second idea that children's and teachers brains needed a break. Teaching has always been recognized as difficult work (which doesn't mean teacher compensation changed because of that perception or their jobs were made easier) and the notion of continuing education for teachers was fairly common by the end of the 1800s. In effect, those who ran schools advocated for an extended break so children could frolic and summer and their teachers could relax, recharge, and take classes on the newest and greatest pedagogical practices.
And basically, so goes NYC, so goes the rest of the country. Mostly. While there are some regional exceptions, summer vacation for students is generally from the middle of June until the day after Labor Day. Teachers are usually off (generally not earning a salary but unable to collect unemployment) from the end of June until the end of August.
Sources and more:
In this post, I get into more about school building construction if that's something that speaks to you.
If you want to get even further in the weeds around school summer vacation, I get into it in this episode of my podcast.
Short read: Agrarian roots? Think again. Debunking the myth of summer vacation’s origins
Long read: Gold, K. M. (2002). School’s in: The history of summer education in American public schools (Vol. 25). Peter Lang Pub Incorporated.