In order to keep the calendar years as close as possible to matching how long it takes the earth to revolve around the sun, we don't exactly have 1 leap year every 4 years (365.25 days/year), it is really 97 leap years every 400 years (365.2425 days/year). We have a leap year in any year divisible by 4, but the exception is that years divisible by 100 are not leap years, however, if the year is divisible by 400 the exception doesn't apply and it is a leap year. So 1700, 1800, and 1900 were not leap years, 2000 was a leap year, 2100, 2200, and 2300 will not be leap years, 2400 will be a leap year, etc. Basically with a leap year every 4 years, the calendar year is slightly longer than an actual year (time for the earth to revolve around the sun), and this makes the equinox start slightly earlier. Then when you skip the leap year in 2100, 2200, and 2300, that makes the equinox start a day later.
In order to keep the calendar years as close as possible to matching how long it takes the earth to revolve around the sun, we don't exactly have 1 leap year every 4 years (365.25 days/year), it is really 97 leap years every 400 years (365.2425 days/year). We have a leap year in any year divisible by 4, but the exception is that years divisible by 100 are not leap years, however, if the year is divisible by 400 the exception doesn't apply and it is a leap year. So 1700, 1800, and 1900 were not leap years, 2000 was a leap year, 2100, 2200, and 2300 will not be leap years, 2400 will be a leap year, etc. Basically with a leap year every 4 years, the calendar year is slightly longer than an actual year (time for the earth to revolve around the sun), and this makes the equinox start slightly earlier. Then when you skip the leap year in 2100, 2200, and 2300, that makes the equinox start a day later.