Wikipedia: Gregorian Calendar
Encyclopedia entry about the Gregorian or New Style calendar, the twelve-month calendar introduced in the Western world in 1582 to replace the less precise Julian calendar.
Perpetual Calendar, The
Article about the process of switching from the Julian to the Gregorian calendar, with a table showing when each nation made the transition.
Summary of the Gregorian calendar, the calendar currently in use in most of the Western world, the mathematical rules it uses, and the history of its slow adoption in the 17th and 18th centuries.
Text of the papal bull proclaiming the Catholic Church's adoption of the Gregorian calendar, signed by Pope Gregory XIII in 1582. In English, French, and Latin.
Gregorian Calendar Definitions
Answers common questions about the Gregorian calendar, the most common calendar in use today, including what years are leap years, when the third millenium really started, and what the 4000-year rule is.