Answer: Julius Caesar
Some extra relevant information:
The Julian calendar, named after Julius Caesar, was instituted by the Roman dictator himself in 45 BCE. In his effort to reform the Roman calendar, Caesar commissioned the astronomer Sosigenes to devise a new system that would accurately align the calendar year with the solar year. The resulting Julian calendar replaced the outdated Roman Republican calendar, which had fallen out of sync with the seasons due to its flawed method of calculating leap years. Sosigenes’ proposed calendar introduced a 365-day year with a leap year occurring every four years. This adjustment ensured that the average length of a calendar year closely matched the solar year, helping to maintain the accuracy of seasonal events over time. Despite some minor discrepancies with the solar year, the Julian calendar was widely used in the Western world for over 1600 years before it was replaced by the Gregorian calendar in 1582.