Five thousand years ago the Egyptians divided the year into twelve months and each month is divided into a certain number of days. Three years out of every four, a year has 365 days in it. In the fourth year, there are 366 days. This is a leap year. Every leap year an extra day is added to the month of February. Normally, February has 28 days in it, but in a leap year it has 29 days.
The purpose of this is to make up the difference between the calendar year and the solar year. It is convenient for us in daily life to go by a calendar year of 365 days. The solar year, which measures time by the movement of the Earth round the Sun, is actually 365 days and about six hours long. In four years, these six hours a year make an extra day, so we have a year with an extra day in it, a leap year.
Calendars sometimes show the positions of planets and stars and the phases of the moon.