Leap yearday is added to the Gregorian calendar used by most of the world.
An ordinary year has 365 days. A leap year has 366 days. The extra day is added to the month of February. In an ordinary year, February has 28 days. In a leap year, it has 29 days.
A year is a leap year if it can evenly be divided by four. For example, 1996 was a leap year. But a year is not a leap year if can evenly be divided by 100, but not by 400. This is why 1700, 1800, 1900 were not leap years, but 2000 was.
Without leap years, the seasons would start one day earlier on the calendar every four years. After 360 years, the 21st of the month of March (the usual start of spring) would come in winter on what would otherwise be the 21st of the month of December.