leap day

Etymology
See leap year.

Noun

 * 1) An extra day intercalated into a year, especially the day intercalated into the Julian calendar every fourth year and into the Gregorian calendar every fourth year excepting centuries not divisible by 400, usually reckoned as February 29th.
 * 2) * 1820 July, Thomas Yeates, "Catalogue of Ancient Eclipses" in The Philosophical Magazine and Journal, Vol. LVI, No. 267, OCLC 1641414, p. 18:
 * The lunar cycle among the Chaldeans was called the saros and sara, from סהרא, sahara, the moon. This cycle it is said contained two hundred and twenty-three synodical months, or eighteen Julian years, ten days, when the same cycle or period contains five leap days, and eleven days when it contains four leap days, seven hours, forty-eight minutes, and one-fourth; in which time all the corresponding new and full moons and eclipses return again.
 * 1) * 1852, Edward Greswell, Fasti Temporis Catholici and Origines Kalendariae, Vol. II, OCLC 457298519, p. 51:
 * We observe then, with respect to the Julian cycle of leap-year, that, though we speak of it commonly as the cycle of the leap-year, it is in reality only the cycle of the leap-day. And this distinction is of much importance, especially to the present question. For the cycle of leap-year according to the Julian rule is one of four years, because the cycle of the leap-day in the Julian calendar is one of four also.
 * 1) * 1852, Edward Greswell, Fasti Temporis Catholici and Origines Kalendariae, Vol. II, OCLC 457298519, p. 51:
 * We observe then, with respect to the Julian cycle of leap-year, that, though we speak of it commonly as the cycle of the leap-year, it is in reality only the cycle of the leap-day. And this distinction is of much importance, especially to the present question. For the cycle of leap-year according to the Julian rule is one of four years, because the cycle of the leap-day in the Julian calendar is one of four also.

Usage notes
To avoid interfering with Rome's religious festivals during his 46 calendar reform, Julius Caesar added no permanent days to the Roman month of February and placed his system's leap day well before the end of the month, following the festival of Terminalia. Initially, no day was added at all. Instead, the fifth day before the 1st of March—reckoned inclusively by the Romans as the "sixth calends of March"—was considered to last for 48 hours instead of 24 and the year was accordingly known as a "bissextile year". Roman inclusive counting initially caused the priests to add leap days every three years instead of four, and Augustus omitted bissextile years for a few decades until the count became accurate again. The doubled day of the Julian calendar was eventually understood as two separate days, but the leap day was taken to be the first 24-hour period (Feb. 24) rather than the second (Feb. 25). In Britain and the United States, no authority has ever altered this day as the extra one added to the year, although Sweden and Finland have both formally enacted legislation to change it to February 29. Regardless, February 29 is popularly considered the "leap day" of leap years and has been since the late medieval introduction of sequential reckoning of the days of months.

Translations

 * Catalan: dia bixest
 * Chinese:
 * Mandarin:
 * Czech: přestupný den
 * Esperanto: supertago
 * Estonian: liigpäev
 * Finnish:
 * French:
 * German:
 * Hungarian:
 * Icelandic: hlaupársdagur
 * Ido: bisextila dio
 * Irish: lá bisigh
 * Japanese:
 * Latin: dies intercalarius
 * Northern Sami: gárgádusbeaivi
 * Russian: високо́сный день
 * Spanish: día bisiesto,
 * Swedish:
 * Thai: วันอธิกวาร
 * Welsh: diwrnod naid