The term for inserting extra time in calendars is "intercalation," which doesn't exactly roll off the tongue. We use "leap year" because each date on the calendar jumps ahead two days of the week instead of one. For example: Christmas 2014 fell on Thursday, and last Christmas fell on Friday. This year, Christmas will "leap" over Saturday and fall on Sunday. read more
February 29 is a date that usually occurs every four years, and is called leap day. This day is added to the calendar in leap years as a corrective measure, because the Earth does not orbit the sun in precisely 365 days. The Gregorian calendar is a modification of the Julian calendar first used by the Romans. read more