The Gregorian calendar is the modern calendar widely used throughout the world. It improved the previously popular Julian Calendar that was off by 11 minutes causing a drift of about 3 days every 400 years.
This drift upset the Christian Liturgical calendar because it caused the Vernal Equinox to fall gradually earlier and earlier, which affected the date of Easter.
The Gregorian calendar was adopted by papal bull decreed by Pope Gregory XIII in 1582.
The Gregorian calendar divides the year into 365 days, adding one more day every year that is divisible by four except for those years exactly divisible by 100, except those divisible by 400.
These are called leap years, and the extra days, added to the end of February, are called leap days.
Months in the Gregorian calendar
The Gregorian calendar is divided into twelve months: