February 29: the extra day added to the Gregorian calendar in leap year.
Why Do We Have Leap Year?Even though the standard calendar year is 365 days, the Earth actually takes 365 days 5 hours 48 minutes and 46 seconds to go completely around the sun. (This is called a solar year.) In order to keep the calendar cycle synchronized with the seasons, one extra day is (usually) added every four years as February 29th. The Julian calendar (established by Julius Caesar in …
Origin of leap day
First recorded in 1590–1600
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019