It’s a leap day in a leap year.
With a leap year comes the Leap Day, today, February 29. As most are aware, a leap year occurs approximately every four years to accommodate alignment with the astronomical year.
An astronomical year lasts slightly less than 3651/4 days. The historic Julian calendar has three common years of 365 days followed by a leap year of 366 days, by extending February to 29 days rather than the common 28. The Gregorian calendar, the world’s most widely used civil calendar, makes a further adjustment for the small error in the Julian algorithm. Each leap year has 366 days instead of 365. This extra leap day occurs in each year that is a multiple of 4, except for years evenly divisible by 100 but not by 400. (Wikipedia)
The first leap year was recorded in 45 B.C. by Julius Caesar when he reformed the Roman Calendar and was later revised with the Gregorian Calendar in 1582. According to the Oxford English Dictionary, the term “leap year” goes back to around 1387 (Oxford: OUP, 1971. Trevisa, Higden (Rolls) IV, 199).