The vernal equinox (around March 21) occurs when the Sun passes from the southern hemisphere of the celestial sphere to the northern hemisphere. At this time, the number of hours of daytime and darkness are the same, with minor latitude corrections due to refraction.
The difference in time between the vernal equinox from one year to the next is called the tropical year. However, the tropical year is not always the same length. This is due to the detailed motion of the Earth, especially the 'wobble' in the Earth's rotation axis known as precession.
Currently, a tropical year lasts for 365.24219 mean solar days. Compare this to the average length of a year as measured with the Julian calendar (365.25 days) and the Gregorian calendar (365.2425 days). The discrepancies between these calendar years and the tropical year means that the calendar slowly drifts, and the date of the vernal equinox changes.