The calendar was changed from the Roman Republican Calendar by Caesar because he was tired of its corruption. Priests back then would exploit the calendar for political reasons, and would add in days, sometimes months, in order to keep favored politicians in office.

The new calendar had an average of 365 ¼ days per year. So to solve this problem, Caesar created a leap year every four years, which meant that every four years, they would repeat February 23. In the Julian calendar, there was never a 29th of February. The year was broken into 12 months: Ianuarius, Februarius, Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November, and December. In 44 B.C. the Romans renamed Quintilis, Iulius, after Julius Caesar, and in 8 B.C. Sextilis was renamed Augustus, after Caesar Augustus.

The major problem with the Julian Calendar was the fact that it was off by 11 minutes every year. The Julian system says a year is an average of 365.25 days, when it takes closer to 365.2422 days for the earth to return to the same point while orbiting the sun, putting the calendar off by 11 minutes, or 1 day every 128 years.