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/astro...
http://www.britannica.com/science/Bab...
http://scienceworld.wolfram.com/astro... https://books.google.com/books?id=5O7...
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http://www.livius.org/articles/concep...
http://www.gutenberg.org/cache/epub/1...
http://www.newworldencyclopedia.org/e...
http://www.wolframalpha.com/input/?i=...
http://www.biblicalcatholic.com/apolo...
http://www.britannica.com/topic/pontifex
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