Leap Years are needed to keep our calendar in alignment with the Earth's revolutions around the sun. It takes the Earth 365 days and 6 hours (a tropical year) to circle once around the Sun.
However, the Gregorian calendar has only 365 days in a year, so if we didn't add a day on February 29 nearly every 4 years, we would lose almost six hours off our calendar every year. After only 100 years, our calendar would be off by approximately 24 days!
Happy Wedding Anniversary to my sister - 20 years today or is that 5 years married!
Enjoy your extra day!