Leap year: Difference between revisions

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In the lunisolar [[Hebrew calendar]], [[Adar 1|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. In the [[Solar Hijri calendar|Solar Hijri]] and [[Bahá'í calendar]]s, a leap day is added when needed to ensure that the following year begins on the [[March equinox]].
 
The term ''leap year'' probably comes from the fact that a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, but the day of the week in the 12&nbsp;months following the leap day (from 1 March through 28 February of the following year) will advance 2&nbsp;days due to the extra day, thus leaping over 1&nbsp;day in the week.<ref>{{citation | first = Douglas | last = Harper | url = http://www.etymonline.com/index.php?term=leap+year&allowed_in_frame=0 | title = leap year | work = Online Etymology Dictionary | date = 2012 | access-date = 15 August 2012 | archive-date = 21 August 2012 | archive-url = https://web.archive.org/web/20120821214540/http://www.etymonline.com/index.php?term=leap+year&allowed_in_frame=0 | url-status = live }}</ref><ref>{{citation |url=http://www.oxforddictionaries.com/us/definition/american_english/leap-year|archive-url=https://web.archive.org/web/20150913171136/http://www.oxforddictionaries.com/us/definition/american_english/leap-year|url-status=dead|archive-date=13 September 2015|title=leap year |website=Oxford US Dictionary | access-date = 6 January 2020}}</ref> For example, [[HalloweenChristmas Day]] (3125 OctoberDecember) will be on a ThursdayWednesday in 2024, FridayThursday in 2025, SaturdayFriday in 2026, and SundaySaturday in 2027, but then will "leap" over MondaySunday to fall on a TuesdayMonday in 2028.<!-- Please do not change this list until after 14 Feb 2024 /at the very earliest/; leaving it OctoberDecember 2024 would do no harm at all. -->
 
The length of a day is also occasionally corrected by inserting a [[leap second]] into [[Coordinated Universal Time]] (UTC) because of variations in Earth's [[rotation period]]. Unlike leap days, leap seconds are not introduced on a regular schedule because variations in the length of the day are not entirely predictable.