The cheapest days of the week to fly on U.S. domestic routes are Tuesday, Wednesday and Saturday, according to FareCompare. On Wednesdays there is a higher ratio of tickets to passengers, prompting many airlines to offer discounts.
Some airlines have fewer planes in the air on Tuesdays and thus fewer passengers, making it a good time to find cheaper flights. Saturdays are less expensive because most people traveling on weekends fly Friday, Sunday or Monday. Fewer seats are taken on Saturdays and are usually sold at steep discounts.
The exception to the Tuesday, Wednesday, Saturday rule is holiday flights, as any day of the week just before a holiday has a dramatically increased fare. To get a discounted flight during the holidays, travelers are encouraged to fly on the day of the holiday itself.