Why does the timing of the vernal equinox differ from year to year?

The equinox does not necessarily occur on the exact same day each year, as the vernal equinox in the northern hemisphere occurs on the 20th or 21st of March, these variable dates are because the Earth year is not exactly 365 days, there is an extra quarter of the day (6 hours) that accumulates Each year, causing the date of the equinox to shift. The planet's orientation toward the sun also changes constantly, adjusting the timing of the equinox.