On this Leap Year Day you can read about the Julian calendar, introduced by Julius Caesar in 46 B.C., which included an adjustment that took into account the Earth’s orbit around the sun. That wasn’t quite good enough, so in 1582 Pope Gregory instituted the Gregorian calendar, the one we still use. You can find the rules and mathematical equation for determining when leap years should occur, a chart comparing different ways of configuring the calendar, and references for further research. Use your extra day wisely.