Why do we have leap year?
Our solar year (the time required for Earth to travel once around the Sun) is 365.24219 days.
Our calendar year is either 365 days in non leap years or 366 days in leap years (Feb 29th inserted).
A leap year every 4 years gives us 365.25 days, sending our seasons off course and eventually in the wrong months.
To change .25 days to .24219, we need to skip a few leap days (Feb 29ths) …. century marks not divisible by 400. So with a few calculations tweek the calendar by skipping 3 of 4 century leap years to average out our calendar year to 365.2425, which is pretty darn close to the solar year 365.24219.