Why is there a Leap Day ever four years? It all starts with Easter.
Easter is also referred to as the Paschal season. “Paschal” is a term coming from the Jewish Passover (Pesach), which took on a new meaning for Christians in the earliest days of the church. Passover was celebrated on the first full moon after the vernal equinox. The 14th day of Nisan in the Jewish calendar was the day of preparation for Passover, when a year-old lamb was sacrificed. Paul, in First Corinthians, made the comparison to Christ, “For Christ, our paschal lamb, has been sacrificed”
The date of Easter was the subject of considerable controversy in the early church, with one group—the “Quartodecimans” or 14th-day proponents—arguing that the crucifixion should be observed on the 14th day of Nisan, and the resurrection two days later, regardless of the day of the week on which Easter fell.
The Council of Nicea settled the matter in A.D. 325 by declaring that henceforth Easter would be observed on the first Sunday after the first full moon following the vernal equinox—the point when the sun crosses the equator and day and night are equal.
Why do Eastern Orthodox Christians celebrate a different date for Easter?
The difference can be traced to the introduction of the Gregorian calendar in the 16th century.
Under the Julian calendar, the year was 365 days long with a leap day added every fourth year. But Caesar’s year was 11 minutes, 14 seconds shorter than the solar year—the time it takes the earth to revolve around the sun.
By the 16th century, Easter was falling 10 days too early and Pope Gregory XIII, in 1582, proposed reforming Julius Caesar’s calendar by dropping 10 days.
In 1752, when England got around to adopting Pope Gregory’s calendar, 11 days had to be dropped to make things come out even. There were reports that people rioted in the streets, demanding their 11 days back.
Today Greek and Russian Orthodox Christians still base the liturgical year on the Julian calendar, and the dates of Easter can vary by several weeks between east and west.
Since Easter is a moveable feast—changing based on the date of the vernal equinox and the first full moon of spring—the date of Easter can fall anywhere between March 22 and April 25.
Easter hasn’t fallen on March 22—the earliest possible date—since 1818, and won’t fall on that date again until 2285. Easter fell on the latest possible date, April 25, in 1943, and will do so again in 2038. Curiously, Easter fell on March 24 only one time between 1875 and 2124, in the year 1940.
Another little know Leap Year fact: Under Pope Gregory’s system, leap days come at the turn of the century only if the year is divisible by 400. The year 1900 not being divisible by 400, people born on Feb. 29, 1896, went for eight years without a “real” birthday. Those born on Feb. 29, 2000, had a birthday that, at twice in a millennium, is the rarest birthday of all.
Those born on Feb. 29 share a birthday with bandleader Jimmy Dorsey, Dinah Shore, Al Rosen (baseball’s most valuable player for 1953) and Herman Hollerith, inventor of punch card accounting—as well as famed Italian composer Gioachino Rossini, honored with his own Google doodle.
Other celebrity Leap Year babies include actors Antonio Sabàto Jr. and Dennis Farina, rapper Ja Rule, and (according to DC Comics) Superman.