When is the best time to buy plane tickets?

Airfares fluctuate constantly. In fact, during the typical 11-month period a flight is listed, the price changes every 4½ days on average, according to CheapAir.com’s annual airfare study.

The optimal time to book depends on where and when you plan to travel. Here’s some guidance:

Domestic flights

When: Between 21 and 105 days in advance; in particular, 54 days in advance.

Why: Airlines tend to price flights higher at first to get a sense of demand, said Patrick Surry, chief data scientist at Hopper, an airfare analysis app. If demand is low, prices fall. If it’s high, prices rise. If you wait too long, seats fill up and the price increases.

Best days of the week to book

When: Wednesday and Thursday

Why: Computers normally control ticket prices using complex algorithms, making it difficult to predict when fares are lowest. However, humans decide when to schedule flash sales — short-lived deals that usually last a day or two — and these often pop up during the week, Surry said.

What to expect: The average savings between the best and worst days of the week to shop is about $10 for domestic markets and about $30 for international markets, according to Hopper.

Anticipate seasonal anomalies

Allow yourself an extra buffer for peak times, such as weekends and holidays.

When to buy: Fall, 47 days in advance; winter, 54 days in advance; spring, 75 days in advance; summer, 76 days in advance

Why: It’s a case of supply and demand.

What to expect: Buying tickets on the best day versus the worst day for each season yields more than $200 in savings, according to CheapAir. However, for some high-demand times, ticket prices are relatively static. For example, domestic plane tickets are expected to hold steady around $300 round trip for Thanksgiving, according to data from Skyscanner, a travel search site. Maximum savings for Christmas and New Year’s Eve can be found in the three to seven weeks leading up to each holiday.

More ways to save

Use a flight comparison tool. Apps and websites such as Hopper, Skyscanner and Airfarewatchdog track fares and can alert you to the best time to buy a ticket for your desired route.

Check social media. Follow travel companies and airlines on Twitter and Facebook for sale announcements.

Ask for a price adjustment. “If you find that the fare drops subsequent to your decision to buy, ask the carrier to match it. Some will, and Southwest and Alaska often do so without change fees,” said Robert Mann, president of R.W. Mann & Co., an airline industry consulting firm. However, airlines will often issue refunds in the form of travel credit.

Be flexible. Keeping your options open can help you save money. Try moving dates around or flying out of different airports.

Schwahn is a writer for NerdWallet, a personal finance website.