Because it takes Earth roughly 365.24 days to orbit the sun, our 365-day calendar doesn't perfectly match the orbit. To correct this, we use leap years, which causes the exact time and date of the equinoxes and solstices to drift slightly each year.
Because the Earth’s orbit isn’t exactly 365 days, these dates wiggle slightly year to year, usually falling on the 20th, 21st, or 22nd of the month. season change dates