Wikimore

Deutsch: Gregorianischer Kalender, Gregorianische Kalender

English: The Gregorian calendar is the most widely used calendar in the world today. It was first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, after whom it was named, on February 24, 1582 by papal bull w:Inter gravissimas. It is a reform of the Julian calendar.

Pope Gregory XIII in an early 17th century engraving.
English: Inscription on the grave of Gregory XIII, St. Peter's Basilica, gregorian calendar

Deutsch: Tafel vom Grab Gregor XIII, Petersdom, Gregorianischer Kalender

Lëtzebuergesch: Gregorianesche Kalenner, Petersdom, Roum.

Português: Rosto da Bula papal "Inter Gravissimas" de Gregório XIII estabelecendo a Reforma do Calendário Gregoriano.

English: The first page of the papal bull "Inter Gravissimas" by which Pope Gregory XIII introduced his calendar.
Seasonal error of Gregorian calendar
English: An Election Entertainment - includes famous "Give us our Eleven Days" protest slogan against the Gregorian calendar at lower right (on black banner on floor under foot).
Deutsch: 400 Jahre Gregorianischer Kalender)

English: Stamp, four hundred years of the Gregorian Calendar
English: Comparing two different definitions of the tropical year (from spring equinox to spring equinox, and time needed by the mean sun for 360° on the ecliptic) with the length of the Gregorian calendar year, allowing for the slowing-down of the Earth's rotation.
English: The date of the northern summer solstice between 1750 and 2250
Look up day of week for any Gregorian date
Deutsch: Ewiger Gregorianischer Kalender ab dem 15. Oktober 1582
In September 1752, Britain changed from Old Style (O.S.) to New Style (N.S.)
This is what common calendars look like today, often with an image or picture representing the month.