Why do seasons start on different days each year?
Quick Answer: Seasons start on different days each year because Earth’s orbit isn’t perfectly aligned with our calendar. The extra fraction of a day each year causes a gradual shift, making equinoxes and solstices move around a bit annually.
Our calendar year is 365 days, but Earth takes about 365.24 days to orbit the Sun. That extra 0.24 day adds up, so we add a leap day every four years to catch up. Still, this adjustment isn’t perfect, causing seasonal start dates to shift slightly.
Also, gravitational tugs from the Moon and other planets cause tiny changes in Earth’s orbit and tilt. These subtle wobbles affect exactly when the Sun reaches the points in the sky that mark season changes.