An equinox occurs twice a year when the title of the Earth's axis is inclined neither away from the sun, nor towards the sun. The word equinox is derived from latin - equi - meaning equal and nox - meaning night. The word carries this meaning because the night and day on the equinox are of equal length - 12 hours each. The vernal equinox occurs around March 20th or 21st and the autumnal equinox occurs six months later in September. This day is also celebrated as the first day of spring and marks a new year for many cultures around the world.
March equinox commemoration
The Christian churches calculate Easter as the first Sunday after the first full moon on or after the March equinox. The official church definition for the equinox is March 21; however, as the Eastern Orthodox Churches use the older Julian calendar, while the Western Churches use the Gregorian calendar, both of which designate March 21 as the equinox, the actual date of Easter differs. The earliest possible Easter date in any year is therefore March 22 on each calendar. The latest possible Easter date in any year is April 25