March 1 or March 20 - why are there two dates for the start of spring in France?

Talk of the ‘start of spring’ this week may have led to confusion if you looked at a calendar. 

Despite the warmer-than-usual weather over the last few days – set to continue well into the week – 2026 calendars will have the start of spring marked as March 20.

Meteorologists however say that spring started on Sunday, March 1, nearly three weeks prior. 

The date was in place long before the recent warm temperatures however, and has not been brought forward due to the mild conditions.

Why two dates? 

The difference comes down to the calendars used.

In the northern hemisphere seasons have traditionally been measured via astronomy, using the solstices and equinoxes. 

The winter and summer solstice are the shortest and longest days of the year, whereas the equinoxes are the midpoint when days and nights are of equal length. 

These dates are variable in the Gregorian calendar (by a couple of days) but are used as an easy way to measure a shift in seasons – the time between the spring equinox and summer solstice is spring, between the summer solstice and autumn equinox summer, etc. 

Calendars will use these traditional dates to mark the seasons, including spring, as part of a cultural phenomena dating back thousands of years. 

Meteorologists however use their own dates, adjusted to fit with weather patterns. 

The meteorological calendar is fixed, with three full months given to each season. Spring runs from March 1 to May 31, summer from June 1 to August 31, etc. 

“In the northern hemisphere, most countries have adopted this rule,” says French state forecaster Météo France. 

Although at first glance using fixed dates corresponding to the start of the month may look simpler, there is scientific logic behind it. 

The summer solstice happens on June 20 - 21, the longest day of the year with the most amount of daylight (clouds permitting). 

However, temperatures do not reach their maximum levels in France until around three weeks later, roughly mid-July. 

The same thing happens with winter – the coldest period of the year falls not on the winter solstice, but (generally) mid-January. 

The meteorological forecast therefore places the ‘peak’ of the two most intense seasons at its midpoint, with spring and autumn fitting either side. 

Having the same fixed dates also helps researchers collect weather information and scientific data that can be compared more easily to historical trends. 

For example, rainfall in February reached well above typical ‘winter’ averages (December 1 - February 28) in several areas of France, a fact made easier to measure by the fixed seasonal dates. 

No correct choice 

There is no ‘correct’ date between the two and usage largely depends on the context.

More people are looking towards the meteorological calendar however, as it matches closer to modern lifestyles including agricultural production, tourism and holiday dates, and modern weather patterns. 

That being said, weather anomalies such as last week’s highs of nearly 30C are becoming more common, and fall outside the norms for both traditional and meteorological spring.