March 20-21 (Northern Hemisphere)
The Spring (Vernal) Equinox is one of two days in the calendar year when day and night hours are almost equal to one another; it is also the first day of Spring in the Gregorian calendar.
The Origin of Equinox Celebrations
The annual seasons happen because of two planetary occurrences:
1) a 23.5° tilt of the earth's axis, which makes the earth rotate like a gyroscope, so it points continuously in a fixed direction-toward the area of space near the North Star
2) the earth continuously revolves around the sun in an elliptical orbit-once every 365.25 of our solar days.
As a result, for half of the year the Southern Hemisphere is more exposed to the sun than the Northern Hemisphere, and for the other half of the year the reverse is true. At noontime in the Northern Hemisphere the sun appears highest in the sky during summer and lowest in the sky during winter.
The Vernal Equinox is almost exactly halfway between the winter and summer solstices, and in the sky the sun appears about midway between its highest and lowest points. It is one of the two times during the year when day and night are almost exactly 12 hours long, and very close to being equal to one another. The other day is the Autumnal Equinox, which usually falls on or near the first day of Autumn (September 22-23 NH / March 20-21 SH) in the Gregorian calendar.
The First Day of Spring has been a time of celebration throughout recorded Western history. Most ancient Mediterranean Pagan religions had some sort of festival at or around the time of the Vernal Equinox. The celebrations were a way to rejoice in the rebirth of nature and to ritually act out this seasonal change, from the symbolic death of the earth in winter to its resurrection through the budding and blooming that came after the equinox. Many of these religions had stories of a man-god, born of a virgin, who was killed and reborn at this time each year, and this day was often connected to the worship of many fertility goddesses with such names as Ostara, Astarte, Eostre, Ishtar, and Ostra. The name for the modern Christian Easter holiday originated from these spring celebrations.
In every culture the Vernal Equinox signals the return of weather that favors humanity's ability to grow agricultural crops, and for this reason we have associated it with the "rebirth" of our ability to perpetuate ourselves. It is understandable why we have chosen to celebrate such a momentous occasion, and why it has come to represent "rebirth" in a variety of contexts. For freethinkers the Vernal Equinox is a great time to celebrate the rebirth of Western art, philosophy, and culture with secular themes after centuries of religious domination. It is also a perfect day to celebrate the accomplishments of intellect, reason, and logic, and a time to renew connections to our intellectual past as well as renew promises to continue striving to enlighten and improve the human condition.
Another freethought holiday that can be celebrated around this time and as an alternative to Easter is the Excitement of Life Week.