The best time to book the cheapest flight is 54 days before you travel, according to new research.
According to CheapAir, ticket prices are at their lowest at this time but the number isn't set in stone as it is just when fares are the cheapest they will ever reach.
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In general, the budget flying website recommends booking between 112 and 21 days before you take off.
It says tickets are expensive when they first go on sale and at the last-minute.
The 54-day rule also doesn't apply if you are travelling to a popular destination, for example New York for New Year's.