18th century perpetual calander gives the date of Easter each year either in the Julian or Gregorian calendar. The back of the calendar gives instructions for finding the date of Easter. The Gregorian calendar (Western calendar, Christian calendar) is the solar calendar in use throughout most of the world, sponsored by Pope Gregory XIII in 1582 as a corrected version of the Julian calendar. The ordinary year is made to consist of 365 days and a leap year occurs in every year whose number is divisible by four, except those centenary years, such as 1900, whose numbers are not divisible by 400. A perpetual calendar can be adjusted to show any combination of day, month, and year, and is therefore usable year after year