Solstices and equinoxes are important terms in dealing with calendars. The following information on definition of solstices and equinoxes is adapted from Britanica Encyclopedia:
Solstices and equinoxes are sort of opposites. They signal changes of season on earth
There are two solstices, summer and winter. They occur in June (20 or 21) and December (21 or 22), respectively. The summer solstice is the longest day of the year; it marks the start of summer. The winter solstice is the shortest day of the year; it marks the start of winter.
Equinox means there is nearly equal amounts of daylight and darkness. The word equinox comes from the Latin words aequus (equal) and nox (night).
There are two equinoxes each year, occurring around March 20–21 and September 22–23. These are the vernal equinox and the autumnal equinox. The former marks the beginning of spring in the Northern Hemisphere and the latter the beginning of autumn or fall in the Northern Hemisphere. The equinoxes are the only time when both the Northern and Southern Hemisphere experience roughly equal amounts of daytime and nighttime.
So then we have:
Vernal equinox (about March 21): day and night of equal length, marking the start of spring
Summer solstice (June 20 or 21): longest day of the year, marking the start of summer
Autumnal equinox (about September 23): day and night of equal length, marking the start of autumn
Winter solstice (December 21 or 22): shortest day of the year, marking the start of winter.
Time and Date website provides the following information on the spring or March equinox:
The March equinox can happen on March 19, 20, or 21. The last time the March equinox was on March 21 (in UTC) was in 2007. It will happen again in 2101.
Due to time zone differences, the equinox may occur a day earlier at locations that are behind UTC. Take the example of mainland United States. While locations following UTC have seen a March 21 Equinox in 2003 and 2007, there is no March 21 equinox in mainland US in the 21st century!
Between 2020 and 2048, March 19 equinoxes will happen every leap year in Central, Pacific, and Mountain time zones in the United States. In the same period, the years between the leap years will see a March 20 equinox.
In 2024, the March equinox fell on March 19 in all time zones in the United States. Another March 19 equinox will happen in 2028, and this will be the earliest spring equinox in the country since 1896.
