This is the AUTUMNAL EQUINOX in the Northern Hemisphere and the Vernal Equinox in the Southern Hemisphere. An equinox in astronomy is the moment in time (not the whole day) when the center of the Sun can be observed to be directly above the Earth’s equator, occurring around March 20 and September 23 each year. Today this moment is 5:18 pm on the East Coast.
The September equinox marks the first day of Mehr or Libra in the Iranian calendar. It is one of the Iranian festivals called Jashne Mihragan, or the festival of sharing or love in Zoroastrianism. The September equinox was “New Year’s Day” in the French Republican Calendar, which was in use from 1793 to 1805. The French First Republic was proclaimed and the French monarchy was abolished on September 21, 1792, making the following day the equinox day that year, the first day of the “Republican Era” in France. The start of every year was to be determined by astronomical calculation, (that is: following the real Sun and not the mean Sun as all other calendars).
A folk tale claims that only on the March (Vernal) equinox day (some may add the September equinox day or may explicitly not), one can balance an egg on its point. However one can balance an egg on its point any day of the year if one has the patience.
For more on egg balancing: http://urbanlegends.about.com/od/errata/a/equinox_eggs.htm
Although the word “equinox” implies equal length of day and night, as is noted elsewhere, this is not true. For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal. Those days are commonly referred to as the “equiluxes” to distinguish them from the equinoxes. Equinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart. This way, you can refer to a single date as being the equilux, when, in reality, it spans sunset on one day to sunset the next, or sunrise on one to sunrise the next