What is the leap day, February 29, which comes only once every four years?
In the solar calendar, a year deserves 365.2422 days.
However, if you set it to exactly 365 days, an error will gradually occur.
So it was corrected by setting a leap year (366 days) only once every four years, but it is not perfect either.
Therefore, in 1582, Pope Gregory XIII devised a way of ”skipping leap years only three times in 400 years.”
This is called the “Gregorian calendar” and is used in most parts of the world.