The Best and Worst Months to Fly, and Why

Summer thunderstorms and winter snow drag on-time rates every year, and the pattern is remarkably stable. Which months run late, how big the seasonal swing is, and why it's weather more than the airline.

By Eric, OnTimeIndex Founder · Last reviewed July 2026

Some months of the year simply run late, and the pattern repeats with enough regularity that it’s worth planning around. Pooling several years of the federal on-time record (2023–2026) makes the seasonal shape clear: it’s driven far more by weather and traffic than by which airline you fly.

Summer and deep winter are the hard months

Two seasons drag on-time rates. Summer — roughly June through August — brings afternoon thunderstorms that build over major hubs and force ground stops and reroutes, on top of the year’s heaviest travel volume. Deep winter — December through February — brings snow, ice, and de-icing delays. Between them sit the shoulder seasons: late spring and, especially, autumn tend to run the cleanest, with settled weather and lighter crowds.

Most of it is weather, not the airline. In the DOT’s delay-cause filings (BTS delay-cause reporting), the seasonal swing shows up in the weather and National Aviation System buckets — thunderstorms and congestion — not in the airline-controlled categories. Switching carriers won’t move a July thunderstorm.

The swing is bigger at some airports than others

This is why a per-airport view beats the general rule. An airport in a temperate, low-storm region might show only a few points between its best and worst months. A Gulf Coast or Midwest hub exposed to both summer convection and winter storms can swing 15 points or more across the calendar. The shape is broadly shared — summer and winter down, shoulders up — but the size is local, and that’s the part worth checking before you commit to travel dates you can move.

Read the year-by-year breakdown, not just the average

A pooled monthly average can be pulled around by one unusually bad season. If a single brutal January sits inside the pool, that month’s average looks worse than a typical January really is. So the honest way to read seasonality is to keep the year-by-year breakdown alongside the pooled figure: a month that’s consistently poor across every year is a real pattern; one dragged down by a single outlier year is a caution, not a rule.

What this is and isn’t

These are historical base rates by calendar month — a reasonable prior for a season at a given airport, not a forecast for a specific date. Weather in any single year is its own event. Use it to choose when to fly when your dates are flexible; it says nothing about which airline or which time of day, which are separate questions.

The Best Month to Fly tool shows the full month-by-month curve for any U.S. airport, with the year-by-year grid underneath. For the daily version of the same question, see Best Time to Fly.

Sources

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Informational only — every figure here comes from historical federal filings, not a prediction of any future flight, and nothing here is professional travel advice.