The system Numa devised was chaotic. Lunar years fall roughly 10.25 days short of the solar year, causing seasons to drift rapidly out of alignment with harvest cycles. Priests periodically inserted an unruly leap month called Mercedonius to catch up, but politicians regularly manipulated its scheduling to lengthen their terms in office or shorten those of rivals.
Julius Caesar dismantled this corruption in 46 BCE. Working with the Alexandrian astronomer Sosigenes, Caesar decoupled the calendar from the moon and anchored it to the solar cycle of 365.25 days. Caesar distributed ten extra days across the existing months, bringing seven of them to 31 days and four to 30 days. He left February at its baseline 28 days, adding one intercalary day every four years. That framework formed the Julian calendar.
| Month Category | Months Included | Standard Duration | Contains Day 28? |
|---|---|---|---|
| Shortest Month (Common Year) | February | 28 days | Yes |
| Shortest Month (Leap Year) | February | 29 days | Yes |
| 30-Day Group | April, June, September, November | 30 days | Yes |
| 31-Day Group | Jan, Mar, May, Jul, Aug, Oct, Dec | 31 days | Yes |
