The early Roman calendar was a mess. Based on lunar cycles and the agricultural year, it originally had only 10 months, starting in March, and totalled 304 days. The winter months were simply not counted, as no farm work was done. This led to festivals drifting out of sync with the seasons.
In 731 BC, King Numa Pompilius added 51 days to create January and February, bringing the year to 355 days. This odd number was chosen to align with the lunar year, but Roman superstition about even numbers added an extra day. All months except February had odd numbers of days; February, with 28, was considered unlucky and a time for purification.
Despite this reform, the calendar still lagged about 11 days behind the solar year. By 200 BC, the discrepancy was so severe that a solar eclipse recorded on 11 July actually occurred on 14 March by modern reckoning. Priests inserted an extra month, Mercedonius, on an ad-hoc basis, but political interference made the situation worse.
Julius Caesar, advised by astronomer Sosigenes, decided to overhaul the system. In 46 BC, already a year with Mercedonius, Caesar added two more months, creating a year of 445 days—the longest in history. This 'Year of Confusion' realigned the calendar with the seasons and introduced the leap year, but the project was nearly derailed by a counting error in Roman maths.
The Julian calendar, with its 365-day year and leap years, became the standard for centuries, though it still had a small drift that was later corrected by the Gregorian calendar.
