The vernal equinox occurs either the 20th or 21st of March; it marks the start of spring in the Northern Hemisphere and fall in the Southern Hemisphere.
An equinox occurs when “nearly” half of the globe is in darkness and half is in light. Equinox means “equal night” in Latin.
Scientists use a meteorological calendar which divides the year into four seasons, each three months in length. In this calendar, spring begins on 01 March each year.
By following the civil calendar and having less variation in season length and season start, it becomes much easier to calculate seasonal statistics from the monthly statistics, both of which are very useful for agriculture, commerce, and a variety of other purposes.
Easter, a Christian holiday, is celebrated on the first Sunday after the first full moon after the 21st of March. (My mother chanted this when I was a child.) Easter can happen as early as March 22nd or as late as April 25th.
In Japan, spring equinox is a national holiday, Shunbun no Hi.
The Persian or Iranian New Year, Nowruz, also begins on the spring equinox. It has been celebrated for more than 3,000 years.
Throughout history, people have celebrated the coming of spring. In Mexico, the ruins of the Mayan city Chichen Itza provide a light show. The pre-Columbine Pyramid of Kukulcán aligns directly with the sun during the equinox. As sunset approaches, shadows resemble the body of a snake moving along its north side.