The Spring Equinox is a solar festival which occurs when the night and day are the same length, hence its name: equi (equal) nox (night). It is celebrated on the day the sun moves into Aries, the first sign of the zodiac. Since this date is astronomically determined, it shifts each year, falling on either March 21, 22 or 23, whatever date the Sun moves into the astrological sign of Aries, symbolized by a ram, the initiatory energy of spring and also a typical sacrifice at this time of year.

Besides being the start of the astrological year, March 25 was also the start of the calender year for many
centuries. Great Britain and the Colonies began the new year on March 25th (Lady Day) until 1752. In other words, March 23 1750 was the day before March 24 1751. This preserved the old belief that the year began on Spring Equinox. The Persians still celebrate the new year on Spring Equinox with the joyous feast of No Rooz.

March was also the first month of the Roman year until the adoption of the Julian calendar in 46 BCE when January and February were added. March is still acknowledged as the first month of the year by the names of our months (September which is actually the ninth month means seventh, October which is actually the tenth month means eighth and so forth). 

As with other holidays, the Catholic Church linked the day with a significant holiday, in this case, the Annunciation of the Blessed Virgin Mary, who conceived Jesus on this day and gave birth to him exactly nine months later on Christmas Day. Her festival was known as Lady Day. Jesus' other parent, St Joseph, also has a holiday near the equinox on March 19th.  ~ Waverly Fitzgerald