The cheapest day to fly isn't always the deal: reading a flexible-date fare calendar

Give a flight search flexible dates and it shows you a calendar or a grid of prices, with the lowest one highlighted. That number answers one question well: which day is cheapest to fly? It doesn't answer the question most people are really asking when they look at it: is this a good price, and should I book it now?

Those are different questions, and mixing them up is how people either wait too long for a fare that was already normal or skip one that was genuinely unusual.

Cheap for the window, or cheap for its dates

Different departure dates have different normal prices. The Sunday after Thanksgiving and the days after Christmas cost more than a quiet Tuesday in early December, every year, because that's when everyone travels. We wrote about why in when to book holiday flights.

So say your window runs from late November to the end of December, and the calendar shows:

• December 1 at $140, where a seat that week usually goes for about $150
• December 28 at $210, where the week after Christmas usually goes for about $380

These figures are made up for illustration, but the shape is real. December 1 is the cheapest day. December 28 is the better deal: it's far below what those dates normally cost, while $140 is roughly what early December always costs.

Which one matters depends on you:

• If any date works, take the cheapest. Whether it's a "deal" is beside the point when the alternative is paying more for a different day.
• If you need the expensive dates (family at Christmas, a wedding, school holidays), the cheapest day in the calendar is useless to you. What you need to know is whether your dates are priced well right now.
• If you're deciding whether to book now or wait, an ordinary price is by definition what those dates usually cost. There's no reason to rush it. An unusual one is the price worth acting on.

Look at enough dates and one will look like a bargain

There's a second, quieter problem with scanning a calendar for the best-looking fare. Prices on any given date wobble from day to day. On a typical day, each date has some chance of sitting near the bottom of its own normal range purely by chance, with nothing meaningful going on.

Suppose that chance is one in ten for each date. Check one date and you'll see a "bottom-tenth" price one time in ten. Check seven dates and the chance that at least one of them is in its own bottom tenth is about one in two. Check thirty and it's about 96 in 100.

Neighbouring dates tend to move together, so the real odds are lower than that arithmetic, but the direction holds: the more dates you scan, the more likely one of them looks unusually cheap by luck alone. A tool that searches a wide window and flags its single best-looking date as a deal will flag something nearly every day.

What we do about both

A Price Hash Score judges each fare against the history of the same route, the same departure month and the same booking window, so "cheap for its dates" is the only kind of cheap it measures. What "26% below typical" means explains that comparison in detail.

When your window spans more than one of those comparisons, we score the cheapest fare in each one we have enough history for, and the alert goes to the one most unusual for its own dates. Because picking the best of several is a second chance at a fluke, the bar rises with the number we compared: a free alert needs 90 when we compared one set of dates, 95 for two and 97 for three. A missed alert costs you one opportunity. An alert that fired because it was the luckiest of three would teach you to ignore the next one.

And if the fare we alert on isn't the cheapest date we found, the email says so and names the cheaper date and its price. You decide which question you're answering. We just make sure you have both numbers. The details are in how the score works and in what our date-shift line compares.

Reading a fare calendar yourself

• Decide which question you're asking first: cheapest day, or good price for the days you need.
• Compare a price with the same dates, not the whole calendar. A Christmas fare that looks expensive next to early December may be the best Christmas price you'll see.
• Be wary of a "deal" picked out of a big grid. If a tool scanned two months of dates to find it, ask what it's being compared against.
• Check what the price includes. The lowest number on a calendar is often basic economy, and bags and seat selection are extra. See is basic economy actually cheaper.

If you'd rather have a fare judged against its own dates than against the cheapest day in the window, watch a route. It's free while we're in beta.