Introduced by Julius Caesar. It was made in 46 BC. 365 days divided in to 12 month. It has been replaced as the civil calendar. Formed by adding ten days to a regular pre-Julian Roman year of 355 days crating 365 days. Two extra days were added to January, August, and December. One for the other ones.
My resorces are http://en.wikipedia.org/wiki/Julian_calendar