Data: MDR Education; Map: Brad Jennings/Axios. Note: Includes public schools. Does not include year-round schools.America's start-of-school season stretches nearly two months.Why it matters: Gone are the days of a shared "back-to-school" moment. U.S. schools reopen on schedules that stretch from mid-July to mid-September, depending on where they are.The big picture: District calendars show nearly half of all schools will open by Aug. 16, according to dates compiled by MDR Education, a data company. Four in five will open before Sept. 1.Zoom in: Geography, state law and local decisions all play a role.Nearly all public schools in the Deep South open before Aug. 16. Almost none in New England do.Districts largely set their own calendars, but some states mandate start dates. In Minnesota, for example, districts generally cannot begin the school year before Labor Day, with limited exceptions.One reason cited for later starts in parts of the Northeast is protecting summer tourism by maintaining a seasonal youth workforce at beaches, camps and other vacation hot spots.Between the lines: Where states don't impose calendar restrictions, districts cite their own reasons for earlier starts, which could include:Building more breaks and days off into the school year.Providing more instructional time before standardized testing.Ending the first semester before winter break.Yes, but: Summer heat often enters the start-date debate, but it doesn't neatly explain school calendars.Some districts and lawmakers argue August classes increase school cooling costs.Others argue schools provide air-conditioned spaces that benefit children during periods of extreme heat.What we're watching: Whether earlier starts ultimately lead to earlier finishes or simply longer school years as districts add more holidays and longer breaks.