Why Do We Have Leap Years?

So it’s February 29th and we have 366 days this year instead of 365- what’s the deal with Leap Years?

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Sources:

http://scienceworld.wolfram.com/astronomy/GregorianCalendar.html
http://www.britannica.com/science/Babylonian-calendar
http://scienceworld.wolfram.com/astronomy/BabylonianCalendar.html https://books.google.com/books?id=5O7SBAAAQBAJ&pg=PA68
http://www.wolframalpha.com/input/?i=lunar+months+in+a+year
http://www.wolframalpha.com/input/?i=lunar+month+in+days
http://penelope.uchicago.edu/~grout/encyclopaedia_romana/calendar/romancalendar.html
http://www.livius.org/articles/concept/pontifex-maximus/
http://www.gutenberg.org/cache/epub/10701/pg10701-images.html
http://www.newworldencyclopedia.org/entry/Pontifex_Maximus#Duties
http://www.wolframalpha.com/input/?i=%28365.25-365.2425%29*%281582-%28-45%29%29
http://www.biblicalcatholic.com/apologetics/a104.htm
http://www.britannica.com/topic/pontifex
http://www.wolframalpha.com/input/?i=convert+february+29,+2016+to+the+julian+calendar
http://www.wolframalpha.com/input/?i=1%2F%28.2425-.24237%29
http://adsabs.harvard.edu/abs/1992JBAA..102…40M
http://www.wolframalpha.com/input/?i=7700*13