The Julian Calendar was the civil calendar that was used prior to the current Gregorian calendar. The Julian Calendar is based on the same 12 months with a leap year every 4 years. It was discovered that this caused the Julian Calendar to gain 1 extra day every 128 years which was corrected by the Gregorian reform in 1582 that removed leap year in the years that are divisible by 100 (but not 400) to compensate for the gain.
Julius Caesar introduced the Julian Calendar as a reform of the Roman Calendar which was adopted in 45 BC and became the predominant calendar used in most of Europe and Amerian settlements until it was replaced by the Gregorian Calendar.
Some Orthodox Churches still use the Julian calendar to calculate holiday dates.