The early Romans used a calendar in which each month had irregular lengths. At the end of every other year an extra month of 22 to 23 days was added. When Julius Caesar became emperor he had to add 90 days to this extra month to align the calendar back to the proper seasons.

After doing so Caesar introduced the Julian Calendar in 46 BC. This new calendar had 365¼ days. The ¼ day was added between February 24 and February 25. This calendar is similar to the modern calendar we use today. It was amended by Pope Gregory XIII in AD1582.