Here is the worst day to fly this summer, according to air travel site data

If you're planning your summer travel, there's one day you'll especially want to avoid flying.

July 9 has been named the most expensive day to fly between June 1 and September 30 by air travel site CheapAir.com. That's because it's the Sunday after the July 4 holiday. Sundays are often the most expensive day of the week to fly, explained CheapAir CEO Jeff Klee.

"Leisure travelers who want to squeeze every last minute out of their summer vacations often choose Sunday afternoon and evening return flights," he told ABC News. "Business travelers who need to be able to report for work or a conference on Monday morning will often choose to come in the day before."

The average flight that day is nearly $100 more than the summer average of $384.

CheapAir crunched price data from the summer to create a calendar of those four months showing which days are cheaper and which days are more expensive. Independence Day itself is the cheapest day to fly in July. If you are determined to fly the weekend after the holiday, Cheap Air recommends foregoing Sunday for Saturday, July 8 or Monday, July 10.

Early June has days with relatively cheap fares, but CheapAir recommends planning vacations in August or September for the best deals. Another way to save money is to be flexible about which days of the week you fly on. Midweek, such as Tuesday and Wednesday, tends to be cheaper.

Lastly, of course, you can save money by booking soon, before the cheap tickets get scooped up.