A Leap Day, February 29, is added to the calendar during leap years. This extra day, also called Leap Year Day, makes the year 366 days long – not 365 days, like a common year.
The next leap day is Saturday, February 29, 2020. The last Leap Day was on Monday, February 29, 2016. Leap days are needed to keep our calendar in alignment with the Earth's revolutions around the Sun.
It takes the Earth approximately 365.242189 days – or 365 days, 5 hours, 48 minutes, and 45 seconds – to circle once around the Sun. This is called a tropical year.
Without an extra, or intercalary, day on February 29 nearly every four years, we would lose almost six hours every year. After only 100 years, a calendar without leap years would be off by approximately 24 days in relation to fixed seasonal days such as the vernal equinox or the winter solstice.
Roman general Julius Caesar implemented the first leap day in his Julian Calendar, which he introduced in 45 BCE. A leap day was added every four years. At the time, leap day was February 24, and February was the last month of the year.
However, adding a leap day every four years was too often and eventually, in 1582, Pope Gregory XIII introduced the Gregorian Calendar. This calendar, which we still use today, has a more precise formula for calculation of leap years, also known as bissextile years.
Leap day as a concept has existed for more than 2000 years and it is still associated with age-old customs, folklore, and superstition.
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