The Gregorian calendar is the most widely used calendar in the world today. It was introduced by Pope Gregory XIII in 1582 as a reform of the Julian calendar, which had been in use since 45 BC.
The Gregorian calendar is a solar calendar, which means that it is based on the position of the sun relative to the Earth. It has 365 days in a common year, and 366 days in a leap year, which occurs every four years. The leap year is calculated by adding an extra day, February 29th, to the calendar.
One of the main differences between the Julian and Gregorian calendars is the method of calculating leap years. The Julian calendar added a leap day every four years, without exception. The Gregorian calendar uses a more precise calculation, where leap years occur in years that are divisible by 4, except for years that are divisible by 100 but not by 400.
The introduction of the Gregorian calendar was met with resistance in some countries, particularly among Protestant nations. The British Empire, for example, did not adopt the Gregorian calendar until 1752, when it skipped 11 days to align with the new calendar.
Today, the Gregorian calendar is used by most countries in the world for civil purposes, including business and government. However, some religious communities, such as the Eastern Orthodox Church, continue to use the Julian calendar for liturgical purposes.