Between 19 March and 22 March
There are two days each the year where the lengths of the day and the night are identical, and these are known as the Equinoxes. They were acknowledged long before the Roman invasion, and subsequent Christianisation of Britain, and continue to be recognised today. The first, occurring in March, is referred to as the Vernal Equinox.
The Equinoxes were as important, historically, as the longest and shortest days, but instead of identifying the height of Summer and Winter, as the solstices do, the Vernal and Autumnal Equinoxes show the midpoints of Spring and Autumn.
By nature knew he ech ascenciounGeoffrey Chaucer, Nonne Preestes Tale
Of th’ equinoxial in thilke toun;
For whan degrees fiftene were ascended,
Thanne crew he, that it mighte nat ben amended.
The Vernal Equinox takes place in Spring and is the moment when the days will become longer than the nights. That meant control of the crops and seasons moved into the hands of the Oak King of Summer, from the Holly King of the Winter, until the Autumnal Equinox. Just like the moon, each King waxed then waned in a cyclic fashion, sharing the year as light then darkness dominated the hours. It is sometimes referred to as Ostara, taken from the name of the goddess Ēostre, who also lent her name to Easter. In Anglo-Saxon Britain, after the fall of the Roman Empire, Christianity and Paganism held an uneasy alliance, and the month around the Equinox was a celebration of both the new life of Spring and the death and resurrection of Christ. Many traditions from the month became part of the current Easter celebrations, and the day of the Equinox itself would be marked with a feast.
The Equinox remains influential over seasons and calendars, including contributing to the date of Easter, which occurs the Sunday after the full moon following the Vernal Equinox.