February 29th...only every 4 years

Just a quick fact...why do we have February 29 only every four years? Although the modern calendar counts a year as 365 days, a complete revolution around the sun takes approximately 365 days and 6 hours. Every four years, an extra twenty-four hours have accumulated, so one extra day is added to that calendar to keep the count coordinated with the sun's apparent position.

A century year, which ends in two zeros, is not a leap year unless it is also evenly divisible by 400. This means that 1600 and 2000 were leap years, and 2400 and 2800 will also be, 1800 and 1900 were not, and the years 2100 and 2200 will not be leap years. To correct a slight inaccuracy that remains (it isn't exactly six hours extra), it has been proposed that years evenly divisible by 4,000 should not be leap years; but this rule has not been officially adopted.

A leap day is more likely to fall on a Monday than on a Sunday. This is because the Gregorian calendar repeats itself every 400 years, which is exactly 20871 weeks including 97 leap days. Over this period February 29 falls thirteen times on a Sunday, Tuesday or Thursday; fourteen times on a Friday or Saturday; and fifteen times on a Monday or Wednesday.

The concepts of the leap year and leap day are distinct from the leap second, which results from changes in the Earth's rotational speed.

Sourced from Wikipedia...