The equinox is either of two points on our celestial sphere where the ecliptic and the celestial equator intersect. There are two times each year when the Sun crosses the equator, and the day and night are of approximately equal length. With this equinox, the nights begin to grow longer than the days and they will continue to move in that direction until the Winter Solstice in December.
Calendars change, but the equinoxes follow a precise pattern. When Julius Caesar established his calendar in 45 BC, he set March 25 as the spring equinox. Between the 4th and 16th centuries, the calendar drifted with respect to the equinox, such that the equinox began occurring on about March 21st.
Solstices mark when the Sun is farthest north or south and the length of time between Sunrise and Sunset is the shortest of the year while the equinoxes mark the equal points in between.
The equinoxes are not fixed points but move westward along the ecliptic, passing through all the constellations of the zodiac in 26,000 years. This motion is called the precession of the equinoxes. And we thought that 5000 year Mayan calendar marked a long period.