The Gregorian Calendar

The Gregorian calendar is the calendar that is used nearly everywhere in the world. A modification of the Julian calendar, it was first proposed by the Calabrian doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 via the papal bull Inter gravissimas. Its years are numbered per the perceived birth year of Jesus Christ, which is labeled the "anno Domini" era. This era was created in the 6th century by Roman monk Dionysius Exiguus.
The number of days in a Gregorian year is the average number of days per year in the time interval of 400 years of the Gregorian calendar. The Gregorian calendar was established in 1582 by the Pope Gregorio XIII and replaced the Julian calendar established by the roman emperor Julius Caesar in the year 46 B.C.
The Gregorian calendar contains regular years (with 365 days each) and leap years (with 366 days each). The rule used to decide if a year is a regular year or a leap year is quite simple. The year is a leap year if it is a multiple of 4, the centennial years excluded. A centennial year will be a leap year if it is a multiple of 400. So the most recent leap years we had are 2000, 2004 and 2008. Note that 2000 is a leap year only because it is a multiple of 400. The year 1900 was not a leap year. The same can be said of the year 2100 (despite the fact they are multiples of 4). The next leap year will happen in 2012