- 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.
- 1600, Philemon Holland translating Livy as The Romane Historie of T. Livius of Padua, Book XLV, Section xliv, 1232:
- 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.
- 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.
- 2000, Frank Morgan, The Math Chat Book, pp. 9–10:
- According to our current calendar, on what day of the week will January 1 fall in the year 3000? in the year 1,000,000? in the year 101,000,000? […] In the thousand years from the year 2000 to the year 3000, there are 365,000 days, plus 250 leap days, minus 10 century leap days omitted, plus 3 leap days restored in the years 2000, 2400, and 2800, for a total of 365,243 days, or 57,177 weeks and 4 days. Since January 1 falls on a Saturday in the year 2000, it will fall four days later on a Wednesday in the year 3000.
- 2002, Josef Eisinger translating Klaus Mainzer as The Little Book of Time, p. 20:
- According to the new leap rule, leap years are years whose last two digits are divisible by 4. To correct for the slightly shorter length of the solar year, 3 leap years are omitted every 400 years, and to that end, leap days are omitted in the secular years whose unit is not divisible by 4. Accordingly, the years 1700, 1800, and 1900 were not leap years, but 2000 was one again.
To avoid interfering with Rome's religious festivals during his 46 bc 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.