The Julian calendar was the 365-day calendar that Julius Caesar made official in 46 B.C. It replaced a calendar based on lunar cycles. The Julian calendar provided for a leap year with an extra day every four years. Thus, the Julian calendar included an average of 365.25 days each year. By 1582 A.D., however, the Julian calendar had become out of step with the seasonal cycle by 10 days. The adjustment, ordered by Pope Gregory XIII in October, 1582, subtracted ten days from the calendar. The new Gregorian calendar removes a leap year every one hundred years except for years divisible by four hundred.
The Julian calendar has no relation to the Julian date format sometimes used in computing.