(CNN) First came the summer camp promotion from the YMCA of Metro Atlanta, crashing like a brick into my inbox June 17.

"Six more weeks of summer," the subject line taunted. "Make 'em fun!"

Didn't the fine people at the YMCA know that the summer solstice had not yet arrived? And still, here they were, telling me and my 4-year-old that we had only six more weeks of summer?!

But, going by the school calendar, they were right. My son starts pre-kindergarten today at our neighborhood school. That's right -- August 5. It's the same for children in cities and towns across the country, including in Phoenix, Oklahoma City, Indianapolis and Monterey, California. Lots of schools join them the following week and all throughout August.

We're not smashing any records here. In Hawaii and parts of Indiana and Arizona, kids have been in class since late July.

Having grown up in New England, where I was still writing letters home from summer camp in late August, I was perplexed and awash in nostalgia-fueled angst. What happened to school starting after Labor Day?

It turns out a lot of parents have the same question, and there are answers.

But first, a short history of school calendars: Kids didn't always have summers off. In fact, summer vacation as we know it is a pretty recent phenomenon. When the public education system started in the 1800s, calendars varied depending on the needs of the community. In cities, schools were open practically year-round, up to 240 days a year. Rural schools, on the other hand, were open for only about five months over two sessions, in the winter and summer. Fall and spring, school was out so children could help harvest the crops and help with planting, said John Rury, a historian of American education at the University of Kansas.

By the late 1800s, a concern for the professionalization of teachers, periodic financial shortfalls and "the ill effect of too much schooling on students' and teachers' health" were among the factors that moved school leaders to eliminate the summer term, said Kenneth Gold, interim dean of education at The College of Staten Island/CUNY and the author of "School's In: The History of Summer Education in American Public Schools."

In the early 20th century, the rural and urban districts came into alignment so pretty much everyone had a 180-day school year that started after Labor Day and ended in June.

Daphne Sashin

OK, so when did August become the new September?

There are more than 12,000 school districts in the country, and all sorts of laws and reasons govern when they can start and who decides. All the education experts I spoke with seemed to agree that through the 1980s, Labor Day still ruled. But by the mid-1990s, especially in the South, districts began to hop aboard the August train. The last time schools started after Labor Day in my current home of Atlanta, it was 1996.

This year, districts in states from Florida to Kansas to California will start in August and end around Memorial Day.

There are still plenty of schools that start after Labor Day. The later date is popular in the Northeast, for one, and in Michigan, Minnesota and Virginia , there are state laws backed by the local tourism industry that prohibit schools from starting before Labor Day unless they have a waiver.

But, even in those states, some schools are starting in August. This year in Virginia, students in Prince William County will start school before Labor Day for the first time. In Minnesota, Minneapolis is one district that receives an exemption to start before Labor Day, saying the traditional calendar "no longer supports what our children need to be successful in the 21st century."

Why start before Labor Day? Do schools hate summer?

I spoke with several education professors and the head of scheduling for Atlanta Public Schools, and they offered several reasons:

An earlier start date gives teachers more instructional time before statewide assessment tests in the spring. Several experts agreed that this is one of the biggest factors pushing calendars back.

Beginning in August allows students to complete the first semester before the December holiday break, rather than taking tests and turning in big projects after two weeks off. Teachers don't have to spend time reviewing material in January when they should be starting new lessons. Those were some of the reasons given by the Los Angeles Unified School District when it moved up its start date in 2012.

Starting early allows for a fall break in September or October and a winter break around February, in addition to breaks around Thanksgiving, Christmas and Easter. Teachers are happier and kids behave better when they have more breaks throughout the year, said Rebecca Kaye, the Atlanta Public Schools policy and governance adviser, who makes the yearly calendar. "Learning is hard work, and teaching is hard work, and people need breaks," Kaye said. "We have gotten feedback from our employees that they need that time."

When you start after Labor Day and end school in June, that last month is simply not taken as seriously. "That end of the year is perceived as being time that is sometimes not used to the maximum value," Kaye said. And in fact, when Atlanta schools ended in June, a lot of kids simply didn't show up after Memorial Day. "Even though we were having school after Memorial Day, people had it in their minds that school ended. It may seem ridiculous, but that's what happened."

Many graduating students and staff members take summer courses at colleges and universities. Ending school around Memorial Day creates fewer conflicts for them.

I guess those reasons trump my nostalgia. But isn't it too hot for school in August? Is that the best use of money?

Wouldn't schools save money by starting later?

That's what some critics argue . They say that earlier dates put extra pressure on schools' air-conditioning systems and that it would be less expensive to have summer vacation during the hottest part of the year.

Atlanta Public Schools analyzed the costs and determined that there would be minimal, if any, savings by shifting the school year. There are people working in the schools pretty much year-round. "Schools are bustling with activities: summer school programs, summer camps and maintenance activities. And you can't just shut down the air conditioning; the technology requires a certain amount of climate-control to protect the equipment," Kaye said.

Fine. Maybe it's not so bad ...

I'll get used to this calendar, Kaye says. She points out that parents like me tend not to be quite as nostalgic for school in late June. Getting out at Memorial Day is nice, and I suppose June is a lovely time to vacation. Unfortunately, it probably means my kids won't be able to attend one of the summer camps in New England that ends a week after our schools have started. She sympathizes with my plight. But she has more pressing concerns.

If it was up to her, she says, she would extend the school year even longer, like the urban schools in the 1800s.

"I would love to have that 220-day calendar, because our kids -- the majority of the students in the system -- if they're not in school, they're not learning."