According to a study carried out by travel website Kayak in the US the best time to book a flight is 21 days before departure.

According to a study of millions of flights booked by Kayak users over a six-month period, the average fare for a domestic flight in the US is $370; the fare drops to its lowest level, $342, when booked 21 days before departure date.

However, when booking an international fare, the best rate can be booked 34 days before departure, when the average fare drops to $997 from $1,016.

The study also shows that for short domestic trips in the US passengers can save up to 16 percent by departing on a Saturday and coming back on a Monday.

For trips longer than a week, passengers can save up to 10 percent by traveling on a Tuesday and returning on a Wednesday.

Short international flights are 21 percent less expensive leaving on a Tuesday and returning the following Wednesday. For longer trips, passengers cans save up to 9 percent if leaving on a Saturday and returning on a Sunday.

A separate study released in the US in January by the Airline Reporting Corp(ARC), a company that handles ticketing transactions, says that the lowest domestic fares can be found six weeks before departure. The ARC carried out the research looking into the purchase price of airline tickets in 2011.

The two studies agreed that airfares begin to increase by 30 percent or more seven days before the departure day.