Leap Day exists to keep the calendar aligned with the astronomical year - the time it takes the Earth to orbit the Sun is actually 365.2422 days, not a neat 365 days.

The rules for determining a leap year are: a year is a leap year if it is divisible by 4, but if the year is divisible by 100, it is not a leap year unless it is also divisible by 400.

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This is why 2000 was a leap year, but 1900 was not.

February 29 is the only date that exists solely in leap years.

People born on this date are often called "leaplings" or "leap year babies."

The modern Gregorian calendar was introduced in 1582 by Pope Gregory XIII to further refine the Julian calendar and keep the vernal equinox on or close to March 21.

Before the Gregorian reform, the Julian calendar added a leap day every 4 years without exception, which caused the calendar to slowly drift out of sync with the seasons over centuries.

By adding a leap day only in years divisible by 4, except for years divisible by 100 but not 400, the Gregorian calendar maintains a much closer alignment with the astronomical year.

Leap years have 366 days instead of the usual 365.

This extra day is added to the calendar to account for the fact that the actual length of a year is slightly longer than 365 days.

The Gregorian calendar is the most widely used civil calendar system in the world today, being adopted by most countries over the course of the 20th century.

Historically, some cultures and civilizations used calendars without leap years, leading to a gradual drift of seasons over time relative to the calendar.

In the Gregorian calendar, February 29 is the date that "resets" the calendar's alignment with the astronomical year, preventing it from drifting over centuries.

Leap years occur every 4 years, except for years divisible by 100 that are not divisible by 400.

This is to account for the fact that the actual length of a year is slightly less than 365.25 days.

While February 29 is the "official" Leap Day, some cultures and traditions celebrate it on other days, such as the Chinese "Farmers' Holiday" which falls on the 6th day of the 2nd lunar month.