Today is the summer solstice, the “first day of summer” in some reckonings. But there are a number of these reckonings — in the U.S., the summer season is often taken to begin with Memorial Day, late in May and about three weeks before the solstice, and the autumn to begin with Labor Day, two to three weeks before the autumnal equinox. So there are often pointless arguments about when a particular season “really” begins.
Here’s a brief summary, using the (somewhat chaotic) Wikipedia survey as a starting point.
A season is a subdivision of the year, marked by changes in weather, ecology, and hours of daylight. Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth’s axis relative to the plane of revolution. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth’s surface, variations of which may cause animals to go into hibernation or to migrate, and plants to be dormant.
Wikipedia gives four kinds of reckoning for the seasons, starting with the meterological [better: month-based]:
Meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year and winter the coldest quarter of the year.
… [But] In 1780 the Societas Meteorologica Palatina, an early international organization for meteorology, defined seasons as groupings of three whole months. Ever since, professional meteorologists all over the world have used this definition. Therefore, in meteorology for the Northern hemisphere, spring begins on 1 March, summer on 1 June, autumn on 1 September, and winter on 1 December.
A month-based scheme is very common for many purposes, since it has the virtue of simplicity. Rainfall and similar phenomena are often reported by month, with the months then grouped into meterological seasons.
(In my part of California, meterological winter is roughly our rainy season.)
Another month-based scheme uses the quarters of the calendar year, with January 1, April 1, July 1, and October 1 as the dividing points. This is not far off the astronomical reckoning (just below), since these dividing points are about ten days after the winter solstice, the vernal equinox, the summer solstice, and the autumn(al) equinox, respectively.
On to the astronomical (sometimes labeled celestial):
The precise timing of the seasons as viewed by astronomers is determined by the exact times of transit of the sun over the tropics of Cancer and Capricorn for the solstices and the times of the sun’s transit over the equator for the equinoxes.
That’s straightforward, but it’s definitely a definition for scientists, since the transits of the sun are not directly experienced by ordinary people.
Then the ecological:
Ecologically speaking, a season is a period of the year in which only certain types of floral and animal events happen (e.g.: flowers bloom — spring; hedgehogs hibernate — winter). So, if we can observe a change in daily floral/animal events, the season is changing.
More generally, we could talk about a natural reckoning for seasons, using natural phenomena in addition to those involving flora and fauna, in particular rainfall, snowfall, hurricanes, fires, etc.
And then the traditional [better: insolational, or sunlight-based]:
Traditional seasons are reckoned by insolation, with summer being the quarter of the year with the greatest insolation [intensity of sunlight] and winter the quarter with the least. These seasons begin about four weeks earlier than the meteorological seasons and 7 weeks earlier than the astronomical seasons.
In traditional reckoning, the seasons begin at the cross-quarter days. The solstices and equinoxes are the midpoints of these seasons. For example, the days of greatest and least insolation are considered the “midsummer” and “midwinter” respectively.
This reckoning is used by various traditional cultures in the Northern Hemisphere, including East Asian and Irish cultures.
Not included in the Wikipedia article are cultural reckonings of the seasons, using culture-specific events or celebrations. Three of the four seasons are reckoned this way in the U.S.: Memorial Day is the end of spring and the beginning of summer; Labor Day the end of summer and the beginning of fall; and Thanksgiving the end of fall and the beginning of winter. Note that this reckoning comes very close to the month-based scheme described above, since Memorial Day is close to June 1, Labor Day close to September 1, and Thanksgiving close to December 1 (when the “Christmas season” begins).
Many people seem to think of Easter (the time of rebirth) as the end of winter and the beginning of spring. By this cultural reckoning, winter is longer than summer and spring is shorter than fall (since Easter comes some time between March 22 and April 25, significantly later than March 1).
So today is the first day of astronomical summer, but already into summer by some of these other reckonings.