Pope Gregor XIII. emended the incorrect Julian calendar in 1582, because the so-called astronomical spring point ran off. A reform was necessary. Traditionally, the order of weekdays remained, but he decided:
Thursday, the 4th of October 1582 was followed by Friday, the 15th of October 1582 by decree .
Not all countries adopted this reform immediately. In the Protestant part of Switzerland, Germany, the Netherlands, and Denmark Thursday, the 18th of February 1700 was followed by Friday, the 1st of March 1700... In Great Britain Saturday, the 2nd of September 1752 was followed by Sunday, the 14th of September 1752. In Sweden Saturday, the 17.02.1753 was followed by Sunday, the 01.03.1753. Russia adopted this reform only after the October revolution, which thus actually took place in November ! Greece decided to adopt the reform only in 1923 ! Still today there are different forms of calendars, for example the Mohammedan.
The Roman emperor Flavius Theodosius the Great, a fervent proponent of the Christianity, made the Christianity to the state religion in the year 391. He bindingly appointed that the year-counting begins with the year of Jesus' birth as the first year. Furthermore, he disposed the Nativity to be celebrated on December, the 25th instead of on January, the 6th as before. - That was a main stroke against the following bigger pagan celebrations around the winter solstice (=midwinter):
1. the celebration of theinvincible sun-god „SOL INVICTUS”, on December, the 25th
2. the celebration of the birth of the Persian God Mithras on December, the 25th
3. the celebration of the wine-god Bacchus or Dionysos, the so-called Bacchanalien
4. the celebration of the saturn-god, the so-called Saturnalien
The Orthodox still unswervingly celebrate the Nativity on Januar, the 6th resp. in the night of January, the 6th to January, the 7th.
Nonetheless, our calendar does not begin with the birth or the death of Jesus. In the 6th century Dionysius Exiguus fixed the date of Jesus' birth a few years too late. Jesus was born earlier. There are hints for the actual Nativity to having occured during the night of January 6 to January 7 of the year -6 and hints for the night to November, the 12th of the year -7 , in which there occured a big Jupiter-Saturn-Conjunction. For sure, the date of Jesus' birth is before the year -4 Common Era .
Since the year zero has never existed, the 3rd millennium began only on 01.01. 2001. Only at that point of time the first two thousand years CE had elapsed. - On the 01.01. 2000 only the whole number was celebrated, which is all right, too.
The week ranks among the oldest calendar concepts and is the oldest undisturbedly established order of time. Due to the different calendar-imperfections, it may sometimes be difficult to determine a date day-exactly. But the error can be corrected if the weekday is given (Heinz Zemanek, IBM Fellow and professor of the University of Technology Wien). With the Millennium-Uhr, also all weekdays B.C. are computed correctly.
Anyone, who wants to get in to calendar-science deeper, create an own calendar or even program one is recommended to have a look at the website Computus by Herbert Metz, who offers a far-reaching calendar-collection and a lot of literature.
H O M E