USA – Nearly every four year, we add a extra day to the end of the shortest month February called leap day. WHY?
This extra 24 hours helps keep our calendar on track with the earths movements around the sun. See the 365 days is a estimate on how long it takes to revolve around the sun, it actually takes 365.2421 days. So that .2421 has to be added up somehow or we would year after year get further behind the actual calendar. That may not seem like much but after decades and centuries that could really look different.
Why February 29? The orbit of the earth is measured by the vernal equinox in March or the first day of spring. March was originally meant to be the start of the year making February the last month, adding the extra day to the last month of the year.
Now we are getting complicated, Leap day doest actually happen every four years. A year may be a leap year if it is evenly divisible by four. Years that are divisible by 100, century years such as 1900 or 2000, cannot be leap years unless they are also divisible by 400.
For this reason, the years 1700, 1800 and 1900 were not leap years, but the years 1600 and 2000 were.
This rule about years divisible by 100 years and 400 years was added with the introduction of the Gregorian calendar in 1582.
SO that was easy right?