🍂When does the season actually start?

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Two definitions, side by side

Neither definition is wrong and neither is official. They answer different questions, and the calculator above will give you either one.

The practical consequence is that a forecaster and a calendar publisher can announce the first day of spring nine days apart and both be telling the truth.

What an equinox is not

The word promises equal night, and the day of the equinox does not deliver it. On the equinox itself daylight is a few minutes longer than darkness everywhere except the poles, for two reasons that have nothing to do with the orbit: sunrise and sunset are timed from the first and last sliver of the Sun’s disc rather than its centre, and the atmosphere refracts the image of the Sun so that it is visible while it is still geometrically below the horizon.

The date of genuinely equal day and night — the equilux — therefore falls a few days before the March equinox and a few days after the September one, and it depends on your latitude, which the equinox does not. That is the whole point of the equinox as a definition: it is one moment for everyone.

The season and the weather are not in step

Midsummer sits at the start of the meteorological summer, not in the middle of it, and the hottest weeks usually arrive several weeks after the longest day. Land and especially ocean keep absorbing heat while the days are still long, so the temperature peak lags the light peak — the same reason the coldest weeks come well after the December solstice, and the reason a coastal town lags further behind than an inland one.

This lag is exactly why meteorologists refused to inherit the astronomical dates. A climate record needs blocks that begin on the same day every year and line up with how the atmosphere actually behaves, and whole months do both.

Where the choice actually matters

Reading a forecast or a climate story. “The warmest spring on record” almost always means March to May, not the astronomical season.

Anything scheduled in the southern hemisphere. June is midwinter for a tenth of the planet, and a seasonal campaign, a school term or a sports calendar written from a northern desk will be exactly half a year out.

Gardening and frost dates. Neither definition helps much: the last frost is a local statistic, and it can arrive weeks after the calendar says spring began.

Counting down. If you are waiting for a specific instant rather than a month, the equinox and solstice dates on this page feed the countdowns, and the days between dates tool will measure the gap between any two of them.

For the other dates on this site that are decided by a rule instead of a number — including when Easter falls, which deliberately uses a fixed 21 March rather than the real equinox, and the clock change dates that move the daylight around inside the season — see the holiday dates group.