Our calender came from the ancient Romans calender called the Julian Calender named after Julius Caesar. The old Roman calender was inaccurate and the number of days in each month were erratic. Julius Caesar solved that by consulting an Alexandrian astronomer, Sosigenes and he suggested they make the calender solely on how the Earth revolved around the sun. They ended up adding ten days to the old Roman calender by putting a few days into certain months. “Two extra days were added to the months Ianuarius (January), Sextilis (August) and December, while one extra day was added to Aprilis (April), Iunius (June), September and November.” (Time and Date.com). They also added a leap year every 4 years like we do. August is named after Augustus.
The problem was, the Julian calender had too many days! It would add up to an extra day every 128 years that wasn’t needed. A man named Aloysius Lilius made a proposal to correct the Julian calender. “His proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years. Lilius also produced an original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.” (Wikipedia). Pope Gregory XIII accepted it and the Gregorian calender is named after him. We use the Gregorian calender today!
Julius Caesar and Sosigenes Aloysius Lilius