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Probably the best place to start is the Wiki article. Two most important aspects is 1/ it is named after its sponsor, Julius Caesar, and 2/ it corrected the calculation of a year to 365.25 days. In doing so, it posited a leap year every four years that simply added a day in February. (Caesar had stolen days from February to make July, named after him, and August, named after Caesar Augustus, 31 days long.)
The Julian Calendar was reformed in 1582 by Pope Gregory XIII to account for the fact that the solar year was just a little short of 365.25 years - in fact 365.2425 days long. After such a long time they had discovered that they were having 'White Sales' in June instead of January.
Thus most of the world now functions under the Gregorian Calendar; Christmas now remains in December! 😉
There are some facets of the Julian Calendar rarely mentioned, factors that make it useful for Bible Prophecy. That brilliant writer, Bob Ireland, has touched on some of these aspects.
The Julian Calendar postulates that in the year 4713BC the lunar cycle [12 months of 29½ days, or 354 days a year] and the solar cycle [365¼ days a year as computed today] and the Roman Indication Cycle [15 years] exactly coincided, and they would run their separate courses for 7980 years, when they would exactly coincide again. The date 4713BC was supposedly set arbitrarily in 1582 by the scholar Joseph Scaliger
7980 works wonderfully in Bible timeline calculations except that it stops short of 8000, my number for new beginnings and the Biblical number of Christ (Messiah). I hypothesize that perhaps it leaves 20 years for the judgement.
Does the 20 years account for the year 0 on the Julian?