The Gregorian calendar, like the Julian calendar, is a solar calendar with 12 months of 28–31 days each. The year in both calendars … See more

The Gregorian calendar was a reform of the Julian calendar. It was instituted by papal bull Inter gravissimas dated 24 February 1582 by Pope Gregory XIII, after whom the calendar … See more

The year used in dates during the Roman Republic and the Roman Empire was the consular year, which began on the day when consuls first entered office—probably 1 … See more

Extending the Gregorian calendar backwards to dates preceding its official introduction produces a proleptic calendar, which should be used with some caution. For ordinary … See more

This section always places the intercalary day on 29 February even though it was always obtained by doubling 24 February (the bissextum … See more

During the period between 1582, when the first countries adopted the Gregorian calendar, and 1923, when the last European country adopted it, it was often necessary to indicate … See more

The Gregorian calendar continued to employ the Julian months, which have Latinate names and irregular numbers of days:
• January (31 days), from Latin mēnsis Iānuārius, "Month of Janus", the Roman god See more