Brief History of the Gregorian Calendar

  As decreed by Pope Gregory XIII, October 4, 1582, was
  followed by October 15, 1582.  Thus ended the 1600-year
  reign of the Julian calendar upon which the Gregorian
  calendar is based, and thus began the calendar which

  DECwindows Calendar uses to measure time.

  Calendars based on sun and moon movement were used even
  by the ancients, but the first reasonably accurate one was
  the 365-day cycle calculated by the Greek Sosigenes.  This
  was the calendar authorized by Julius Caesar in 46 B.C. The
  Julian calendar (not to be confused with the Julian period;
  see below) had 3 years of 365 days each, followed by a fourth
  year of 366 days.

  The 365-day cycle was more accurately defined in 730 AD
  by the Venerable Bede, an Anglo-Saxon monk, who shortened
  the time by 11 minutes, 14 seconds.  This accumulates to a
  whole day's error every 128 years, or a little more than 3
  days every 400 years.  This being the Dark Ages, nothing was
  done to adjust the Calendar, despite Roger Bacon sending a
  note to Pope Clement IV, informing him of the drifting of the
  date for the vernal equinox.  Later, Pope Sixtus IV did become
  convinced that another reform was needed and called the
  German astronomer Regiomontanus to Rome to advise him.
  Unfortunately, Regiomontanus died of the plague shortly
  thereafter and the plans died with him.

  Thursday, October 4, 1582 was the next time the calendar
  was adjusted.  This last day of the Julian calendar was
  followed by Friday, October 15.  So began the Gregorian
  calendar that we use today, named after Pope Gregory XIII.
  He commissioned the mathematician Father Christopher
  Clavius, S.J., to do the necessary calculations, having been
  authorized to reform the calendar by the Council of Trent in
  1545.

  The Vatican librarian Aloysius Giglio provided a formula for
  long-range accuracy.  He suggested that every fourth year
  be a leap year, except for century years that are not divisible
  by 400.  Thus 1700, 1800, and 1900 would not be leap years,
  but 2000 would be, because 2000 is divisible by 400.  This
  rule eliminated 3 leap years every 4 centuries, making the
  calendar sufficiently correct for most ordinary purposes.

  Additional topics:

  *   Political Acceptance in Europe

  *   Political Acceptance World Wide

  *   Religious Acceptance Worldwide

  *   Julian Period

  *   History and DECwindows Calendar

  *   Overview of Calendar