The airfare shopping site CheapAir.com monitored more than 4 million fares in 2013 to answer the age-old question: How far in advance should you book your plane ticket? 
 
 For the average domestic flight, booking 54 days in advance will offer the cheapest fare. 
 
 But CheapAir's CEO Jeff Klee told Business Insider that travelers should worry more about a window of time than an exact number of days. When the site studied the cost of a flight everywhere from 320 days to one day before departure, it found the prime window for purchasing a ticket sits between 29 and 104 days before takeoff. CheapAir.com 
 
 "You should monitor prices actively in that period to get a sense of the market," Klee said. "When you see a good fare, grab it that day." 
 
 If the airfare drops even further in that prime window, CheapAir maintains a policy to refund travelers up to $100. He said airlines have designated between 15 and 20 different fares for the same seat on a flight, based on a variety of factors. 
 
 "The single biggest factor in a price increase is how full a flight is. You should also consider how many flights travel the route you're looking for," Klee added. "People are surprised how little mileage has to do with the calculation of airfare." 
 
 "The worst thing is to book a ticket too late, but the next worst thing is to book too early when multiple routes have yet to be planned, and you have fewer options to fly somewhere," he said. 
 
 At some points in the year, flying from Los Angeles to San Francisco costs more than flying from Los Angeles to London because fewer flights are offered on the first route. The fewer available options to fly to a certain place, the more expensive it will be to take that flight. Also, the fuller a plane gets, the more expensive the seats will become, Klee said. 
 
 For international flights, you'll want to purchase tickets even farther in advance. Travelers going to Africa should book the earliest, at 166 days in advance, while those heading to the South Pacific can book much closer to their departure, at an average of 70 days in advance. 

 Departure Day 
 
 Table 1: Summary of best and worst days to buy, depart, and return 
 
 To find the cheapest day to fly, Hopper looked at about 11 thousand markets from US origins - 7,500 domestic and 3,500 international - that had a popularity of at least 20 searches per day since January 1, 2013. If we average all domestic destinations together, Tuesday is the cheapest departure day, with Sunday the most expensive. Interestingly, Sunday is the least popular day to travel, with Tuesday a close second. Using the same averaging methodology, the best day to travel for international flights is actually Saturday (Figure 1), which is also one of the most popular days to travel. 
 
 However, pricing and search behavior varies widely between markets, so averaging can be misleading. For comparison, we counted how many markets saw their lowest prices on each day of the week, and the answers were different. For both domestic and international travel, more markets have their lowest prices on Wednesdays (Figure 2) and their most expensive prices on Sunday. Savings for leaving on the best day compared to the most expensive day range from $40 to $50 for domestic markets and $40 to $60 for international markets. 
 
 Figure 1: Popularity and average price savings (vs. most expensive departure day) for domestic and international markets 
 
 Figure 2: Number of markets with the lowest departure price on each day of week, and average price savings (vs. most expensive departure day) for domestic and international markets 
 
 Return Day 
 
 On average, Tuesday is the cheapest day to return from domestic travel and Monday is the best day to return from international travel (Figure 3). However, as was the case for the best departure day, the cheapest days are different when the analysis counts the number of markets by cheapest return day instead of looking at the average. For both domestic and international destinations, the majority of markets had their cheapest price on Monday, Tuesday or Wednesday, with Tuesday being best for domestic and Wednesday for international markets (Figure 4). The most expensive day to return is Friday. When returning on the best day, potential savings are between $40 and $60 for both domestic and international trips, as compared to booking on the worst day. 
 
 Figure 3: Popularity and average price savings (vs. most expensive return day) for domestic and international markets 
 
 Figure 4: Number of markets with the lowest departure price on each day of week, and average price savings (vs. most expensive return day) for domestic and international markets 
 
 Booking Day 
 
 One of the tips that shows up often is to shop for flights on a Tuesday (1) after airlines change their pricing. Looking at average pricing we found that for domestic flights the cheapest tickets actually show up on Thursday, and for international flights Saturday and Sunday are the cheapest to book on (Figure 5). When counting the number of markets that see the lowest booking price for each week day, Hopper found that Thursday is the cheapest day to book for the majority of domestic and international markets (Figure 6) and the weekends are when tickets are the most expensive. However, while the difference in pricing from day to day can be significant in select markets, on average the savings are only about $10 for domestic markets and $30 for international markets. 
 
 Figure 5: Popularity and average price savings (vs. most expensive booking day) for domestic and international markets 
 
 Figure 6: Number of markets with the lowest departure price on each day of week, and average price savings (vs. most expensive booking day) for domestic and international markets 
 
 For any tip that is true in aggregate it is possible to find a market that is the exception, but you can monitor your markets of interest using Hopper’s tools. 
 
 1.http://www.farecompare.com/travel-advice/tips-from-air-travel-insiders/ 
 
 Data and Methodology 
 
 The data presented in this analysis comes from Hopper’s combined feed of Global Distribution Service (GDS) data sources which includes about 10 million queries and 1 billion trips per day. Demand is represented as the number of queries not actual ticket purchases, and is calibrated across all GDS sources for each market. Deal fares are represented by the 10th percentile prices. For example if the 10th percentile price is $800 dollars it means that only 10% of trips are priced at or below this price. 
 
 For this analysis, we looked at about 11 thousand markets worldwide, 7,500 domestic and 3,500 international, that had a popularity of at least 1,000 searches per week since January 1, 2013. We used two methodologies to find cheapest days to depart, return, and purchase. First we averaged flight prices to find cheapest days, and second we counted the number with cheapest days for each day of the week.
– When should you buy your next plane ticket? According to a new fare study from Hopper, Thursday is the best day to book both domestic and international tickets. But keep in mind that it won't save you much: There's just a $10 difference between the best day to book and the worst day (Saturday) for domestic tickets. For international tickets, there's a $25 difference, and the worst day to book is Sunday. More from the study: Best day to depart: Wednesday, for both domestic and international trips. Worst day to depart: Sunday, also for both types of trips. Best day to return home: Tuesday (domestic) or Wednesday (international). Worst day to return home: Friday (both). Another recent study found that, on average, you'll get the cheapest fare if you book 54 days in advance of your flight, Business Insider reports. More importantly, the best window for cheaper tickets is between 29 days and 104 days prior to departure; an expert suggests you monitor prices during that period "to get a sense of the market," then grab a good fare when you see it.