(NEW YORK) -- Summer is always an expensive time to fly, but not all days are created equal.
July 9 is the No. 1 day to avoid, according to data from CheapAir.com. The booking site analyzed hundreds of thousands of summer fares and found that Sunday to be most expensive day of the season to fly.
While average airfare for the summer is $384, according to Jeff Klee, CEO of ChepaAir.com, the average fare on July 9 is nearly $100 more.
Sundays in general are expensive, Klee said, because they're popular with both business and leisure travelers.
"Leisure travelers who want to squeeze every last minute out of their summer vacations often choose Sunday afternoon and evening return flights. The already higher priced flights tend to sell out their lower fares faster," he said.
He added: "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."
July 9 is particularly popular because it's the Sunday following July 4, which falls on a Wednesday this year. Many travelers will look to make an extended weekend out of the mid-week holiday.
Rick Seaney, CEO of FareCompare, said there's another reason July 9 may be so pricey.
"July domestic prices [for flights] are the most expensive behind Christmas and Thanksgiving," he said.
There is some good news for fliers, according to Klee.
"At the moment, 2017 summer fares are tracking slightly cheaper than they were last year at this time, about $20 less per ticket," he said.
But, he warns, prices are likely to climb as summer draws near.
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