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If you want to save money on flights within the U.S., there's one simple trick that works just about any time of year: Fly on a Tuesday or a Wednesday.

How much can this save?

You can expect to save 35 percent on average for domestic trips by flying on Tuesday or Wednesday. I am not a big fan of averages in airfare analysis, as they typically don't reflect much of a relationship to your next trip. But the trend for substantial savings by flying midweek days vs. Fridays or Sundays (the most expensive days to fly) is so stark it's worth further exploration.

When I say you could save an average of 35 percent, it could be less or sometimes a lot more; it depends on how early you shop and whether you catch a sale. Airline ticket prices are based on so many factors that prices can and often do change from day to day, even multiple times in a single day. It also matters where you're flying to and from, as well as whether you choose nonstop flights or even the time of day you fly.

Why the big price difference between weekdays and Friday/Sunday flights?

It's a popularity contest that boils down to supply-and-demand. Vacation fliers like to maximize leisure travel by jetting off on Friday and returning on Sunday, while business travelers work it in reverse, departing Sunday night or Monday morning to get a jump on meetings and returning home Friday to savor their weekends.

Hardly anyone wants to fly on a Tuesday or Wednesday because of jobs or the kids being in school, but the airlines still have to fill those midweek seats, so they make them cheap.

What's the next-best day?

Try flying on Saturday, which is often nearly as cheap as Tuesday or Wednesday. Or, fly on just one of the cheaper days - Tuesday or Wednesday - for your next trip and you'll still reap half the savings.

How many nights do I have stay?

Most cheap Tuesday/Wednesday fares have a minimum-stay requirement of at least two nights so you probably won't get the full savings on a quick, overnight trip.

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