Tomorrow is February 29th, a date that only occurs once every four years.
Because Earth does not rotate the sun in exactly 365 days, calendars that have the same number of days in each year drift over time. By inserting an additional day or month into the year, the drift can be corrected.
Each calendar throughout the world seems to have some form of a leap year, with the Julian calendar adding it in 8 AD. The Chinese and Hebrew calendars add an extra month, with the Hebrew calendar adding it seven times every 19 years.
Fun little fact - in Ireland and Britain, it is traditional that women may only propose marriage in Leap Years. While my parents are from neither place, my mom proposed to my dad on February 29th many years ago!
