2020 has an extra day because it is a leap year. This year has 366 days instead of 365. This year also will not begin and end on the same day of the week, as a “normal” non–leap year does. The extra day is February 29 – a day added nearly every four years to the calendar year.
Why? The short answer is that try as we do to control time, we need to adjust to keep our calendar aligned correctly with the astronomical seasons. The Earth’s orbit around the Sun takes approximately 365.25 days, so without this extra day, our Gregorian calendar would get out of sync.
A leap year is also known as an intercalary year or bissextile year and a year that is not a leap year is a common year.
This is for the Gregorian calendar. It’s a lot more complicated if we get into other calendars. In the lunisolar Hebrew calendar, Adar Aleph, a 13th lunar month, is added seven times every 19 years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons. Complicated. In the Bahá’í Calendar, a leap day is added when needed to ensure that the following year begins on the March equinox.
A long time ago, Leap Day was known as “Ladies Day” or “Ladies’ Privilege,” as it was the one day when women were free to propose to men. One tradition today is called Sadie Hawkins Day which sometimes applies to February 29 and allows ladies to ask men for a date or dance.
Two questions that puzzled me about leap years: 1) Why is it called a “leap” year? 2) What happens if your birthday is on February 29 and you only have that birthday every 4 years?
In the Gregorian calendar, a fixed date normally advances one day of the week from one year to the next. January 1 this year was on Wednesday and normally the following year it would be on Thursday. But in the 12 months following the leap day (March 1 – February 28 of the following year) the day will advance two days due to the extra day. So, next year January 1 and the other days will “leap” over one day in the week. January 1 will leap over Thursday and be on Friday next year.
A person born on February 29 is sometimes called a “leapling” or a “leaper.” In common years, they usually celebrate their birthdays on February 28, but it could be celebrated on March 1 since that is the day after February 28. Though a leaper might claim to be only a quarter of their actual age (by counting only their leap-year birthday anniversaries). For legal purposes, these birthdays depend on how local laws count time intervals.
At the end of 2016, there was a “leap second” added to the year to correct the length of a day into Coordinated Universal Time (UTC). This tiny insertion adjusts because of variations in Earth’s rotational period. Leap seconds are not regular things because variations in the length of the day are not entirely predictable.
And though you won’t hear anything like the madness that surrounded the Y2K bug, leap years can present computing problems if the 366th day or February 29 is not handled correctly.