— -- Summer is always an expensive time to fly, but not all days are created equal.
July 9 is the No. 1 day to avoid this year, according to data from booking site CheapAir.com. It analyzed hundreds of thousands of fares for the coming summer and found that Sunday is most expensive day of the week to fly.
While average airfare for the summer is $384, according to Jeff Klee, the CEO of CheapAir.com, the average fare on July 9 is nearly $100 more.
Sundays in general are more expensive, he 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 midweek holiday.
Rick Seaney, the CEO of FareCompare, said fares are usually up for the whole month. "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.