The Northern Hemisphere's spring — or vernal — equinox can land between March 19 and 21, depending on the year. Its fall – or autumnal — equinox can land between Sept. 21 and 24.

Accordingly, churches in the West observe it on the first day of the full moon that occurs on or following the Spring equinox on March 21., it became a movable feast between March 21 and April 25.