March is the third month of what calendar?

The calendar that recognizes March as its third month is the Gregorian calendar, the international standard for civil use today. This solar calendar divides the year into 12 months, with March following January and February, and typically has 365 days, with a leap day added to February in a leap year.

The Gregorian calendar was introduced by Pope Gregory XIII in 1582 through a papal bull called "Inter gravissimas." It was a reform of the Julian calendar, which had been in use since 45 BCE. The Julian calendar, implemented by Julius Caesar, assumed a solar year of exactly 365.25 days, which was an overestimation by about 11 minutes per year. Over centuries, this slight inaccuracy caused the calendar to drift, leading to the March equinox occurring well before its nominal March 21st date. This drift particularly complicated the calculation of Easter, a key Christian holiday.

To correct this accumulated error, Pope Gregory XIII's reform adjusted the average calendar year to 365.2425 days, more closely aligning it with the actual tropical year. A significant part of the reform involved modifying the leap year rule: while a leap year generally occurs every four years, years divisible by 100 are not leap years unless they are also divisible by 400. To immediately rectify the existing discrepancy, ten days were skipped in October 1582, with Thursday, October 4, 1582, being followed directly by Friday, October 15, 1582. While initially adopted by Catholic countries, Protestant nations and others gradually transitioned to the Gregorian calendar over subsequent centuries.