!!! Overview
[{$pagename}] is defined in [RFC 3339] as In the Gregorian calendar, a year which has 366 [days]. \\ A [{$pagename}] is a [year] whose number is divisible by four an integral number of times, except that if it is a centennial [year] (i.e. divisible by one hundred) it shall also be divisible by four hundred an integral number of times.

[{$pagename}] containing one additional [day] (or, in the case of lunisolar calendars, a [month]) added to keep the calendar [year] synchronized with the astronomical or seasonal year. Because seasons and astronomical events do not repeat in a whole number of [days], [calendars] that have the same number of [days] in each year drift over time with respect to the event that the year is supposed to track. By inserting (also called intercalating) an additional [day] or [month] into the [year], the drift can be corrected. [1]

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [Leap_year|Wikipedia:Leap_year|target='_blank'] - based on information obtained 2017-10-14-