Prices change from day to day, even minute to minute. Scouring multiple websites for the best deal can be overwhelming. And after you book, there’s no guarantee that you got the best price.

“You just don’t know when to pull the trigger. It’s not like buying anything else I can think of,” said George Hobica, founder of Airfarewatchdog.com.

Harriet Levy paid $179 for a recent round-trip flight on American Airlines between New York and Fort Lauderdale, Fla. Sitting just one row behind her, Shirley Harrison paid $215. A few rows back, Ellis and Dianne Traub paid $317 each. There were at least 12 fares on the flight, ranging from $169 to $360.

“There’s no rhyme or reason to it,” Harrison said.

Fares can fluctuate significantly in just a few hours. One Delta flight from New York to Los Angeles jumped from $755 to $1,143 from a Friday to Saturday in late April, then fell to $718 on Sunday.

The flight was one of a dozen The Associated Press tracked over three months for a July 16-22 vacation. The No. 1 finding: Avoid booking tickets on weekends. It’s the most expensive time to buy.

There’s no way to guarantee the best fare. But before booking, travelers should heed this additional advice:

— Book on Tuesday, Wednesday and Thursday. That’s when airlines most often offer sales.

— Buy in advance, but not too early. The best time is four to six weeks before traveling. In general, prices for any given flight are highest eight to 10 weeks and two to three weeks in advance.

— Embrace social media. Airlines are giving more benefits, like exclusive sales, to travelers who interact with them on Twitter and Facebook. Those specials are often gone within hours.

— The so-called discount airlines — JetBlue, AirTran, Southwest and Frontier — adjust their fares less frequently than other airlines, so you can feel more confident locking in a price. But their prices aren’t always the lowest. Researching multiple airlines’ fares is the only way to get a good deal.

It wasn’t always this complicated.

Before the airlines were deregulated in 1978, airfares were approved by the government. Prices were consistent and printed in timetables. But because air travel hadn’t been embraced by Americans and was something of a luxury, planes were often less than half full.

Deregulation allowed airlines to set fares and routes on their own. In the early ’80s, American Airlines started selling empty seats at a steep discount. The aim was to fill up planes and compete with new discount airlines such as PeoplExpress.

To prevent business travelers from trading down to cheaper tickets, the airlines required that fliers buy two to three weeks in advance and spend a Saturday night at their destination. The discount airline ticket was born.

Today, sophisticated computer programs analyze travel data and set a range of ticket prices so airlines can get the most money out of each flight. Prices are generally higher for seats sold at the last minute, which are usually bought by business travelers. Refundable tickets tend to be the most expensive, and cheap fares come with lots of restrictions.

Other factors go into the price, too: How fast are tickets selling compared with previous years? Is there a conference or special event in the destination city? Fares are often adjusted to match other airlines’ prices.

But there’s still a great deal of human intervention.

“To a degree, it’s trial and error,” said Greg Aretakis, vice president of revenue production at Frontier Airlines.

There can be as many as 20 prices on any given flight. Airline executives say that all that helps them boost revenue by 3 to 6 percent. If they price tickets too low, the airline can lose money. If prices are too high, seats go unsold.

“If that seat goes out empty, we can’t put it on the shelf and sell it the next day,” said American Airlines spokesman Tim Smith. “A seat that goes unfilled is like a banana that instantly spoils on takeoff.”