When does spring start? Here’s why each season begins twice.

Every year, weather forecasters welcome the arrival of spring on the first of March—while others contend that the spring really begins a few weeks later with the equinox, which falls on March 19, 20, 21. So who is right about when the seasons begin and end?

It depends on why you’re asking. Seasons are defined in two ways: astronomical seasons, which are based on Earth’s position as it rotates around the sun, and meteorological seasons, which are based on annual temperature cycles. Both divide the year into spring, summer, fall, and winter—yet with slightly different start and end dates for each. Here’s what they mean and how to tell them apart.

Astronomical seasons

People have always looked to the skies to determine the season. Ancient Rome was the first to officially mark those seasons with the introduction of the Julian calendar. Back then, the seasons began on different days than the modern era because of discrepancies with the Gregorian calendar used primarily today. Now, the start of each astronomical season is marked by either an equinox or a solstice.

Equinoxes are when Earth’s day is split almost in half. They occur every six months in the spring and fall, when Earth’s orbit and its axial tilt combine so that the sun sits directly above the Equator. On an equinox, roughly half the planet is light while the other half is dark. As the new season progresses, the sun’s position continues to change—and, depending which hemisphere you live in, the days will get progressively lighter or darker until the arrival of the solstice.

Solstices mark the brightest and darkest days of the year. They are also driven by Earth’s tilt and mark the beginning of astronomical summer and winter. When the Northern Hemisphere is tilted toward the sun, it is brighter and feels like summer—while, at the same time, the Southern Hemisphere is tilted away from the sun, plunging it into a dark winter.

But this method of measuring the seasons presents some challenges. The solar year is approximately 365.2422 Earth days long, making it impossible for any calendar to perfectly sync with Earth’s rotation around the sun. As a result, astronomical seasons start on slightly different days and times each year—making it difficult to keep the climate statistics that are used in agriculture, commerce, and more. That’s why weather forecasters and climatologists turned to meteorological seasons instead.

Meteorological seasons

Since at least the 18th century, scientists have sought better methods of predicting growing seasons and other weather phenomena. Over time, that gave rise to the concept of meteorological seasons, which are more closely aligned with both annual temperatures and the civil calendar.

Meteorological seasons are far simpler than astronomical seasons. They divide the calendar year into four seasons that each last exactly three months and are based on the annual temperature cycle. Winter takes place during the coldest three months of the year, summer in the hottest three months, and spring and fall mark the remaining transition months.

In the Northern Hemisphere, that means the start date for each season is March 1 (spring), June 1 (summer), September 1 (fall), and December 1 (winter). In the Southern Hemisphere, the seasons are reversed; spring begins in September, summer in December, fall in March, and winter in June.

The consistency of meteorological seasons allows meteorologists to make the complex statistical calculations necessary to make predictions and compare seasons to one another. “Dealing with whole-month chunks of data rather than fractions of months was more economical and made more sense,” climatologist Derek Arndt told the Washington Post in 2014. “We organize our lives more around months than astronomical seasons, so our information follows suit.”

So when is the first day of spring? It isn’t March 1 or the spring equinox—it’s both.