The Gregorian calendar is today's internationally accepted civil calendar and is also known as the "Western calendar" or "Christian calendar". It was named after the man who first introduced it XIII – Pope Gregory XIII – in February 1582.
The calendar is strictly a solar calendar based on a 365-day common year divided into 12 months of irregular lengths. Each month consists of either 30 or 31 days with one month consisting of 28 days during the common year. A leap year usually occurs every four years which adds an extra day to make the second month of February 29 days long rather than 28 days.
The Gregorian calendar reformed the Julian calendar because the Julian calendar introduced an error of one day every 128 years. The introduction of the Gregorian calendar allowed for the realignment with the equinox, however a number of days had to be dropped when the change was made.
The Gregorian calendar was first adopted in Italy, Poland, Portugal and Spain in 1582. The Gregorian reform consisted of the following changes:
The Julian calendar is currently (between the years 1901 and 2099) 13 days ahead of the Gregorian calendar (because too many leap years were added). The Gregorian calendar is off by about 1 day every 3236 years.
The Gregorian calendar would not be adopted until much later in Great Britain and America. It wasn’t until September 1752 that 11 days were dropped to switch to the Gregorian calendar.
Sweden and Finland had a "double" leap year in 1712. Two days were added to February–creating a date of February 30, 1712. (This was done because the leap year in 1700 was dropped and Sweden's calendar was not synchronized with any other calendar. By adding an extra day in 1712, they were back on the Julian calendar.)
Japan replaced its lunisolar calendar with the Gregorian calendar in January 1873, but decided to use the numbered months it had originally used rather than the European names. The Republic of China originally adopted the Gregorian calendar in January 1912, but it wasn’t used in China due to warlords using different calendars. However, the Nationalist Government formally decreed the adoption of the Gregorian calendar in China in January 1929.