This is fairly easy to find out as you need to call the school where your child will attend and ask the secretary or principal when the first day of school is schedule.
Also, you might ask the school secretary or principal to send you a copy of the school calendar for the year. This calendar will include such items as the end of each grading period, vacation days, teacher work days, and the last day of school. You might find having this calendar helpful in planning your family's activities and events for the school year.
School year calendars vary across the country. I suggest contacting the school directly to learn more about its start date. I've linked below to SchoolFinder, a directory of U.S. schools that includes the schools' telephone numbers.
I've also included some Back to School resources you may also find helpful. Thanks for asking!