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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 months following the leap day (from 1 March through 28 February of the following year) will advance two days due to the extra day, thus leaping over one 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, 1
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.
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