Here's why:
* Solstices mark the longest and shortest days of the year. The summer solstice has the longest day and shortest night, while the winter solstice has the shortest day and longest night.
* The Earth is tilted on its axis. This tilt is the reason we have seasons. During the solstices, one hemisphere is tilted towards the sun, experiencing longer days, while the other is tilted away, experiencing shorter days.
Equinoxes are the times of year when the day and night are roughly equal. This occurs twice a year, around March 20/21 (vernal equinox) and September 22/23 (autumnal equinox).
