The Julian Calendar was implemented 46 B.C. by Gaius Julius Caesar. It has been declared obligatory for all Christians in 325 A.C. by the council of Nicäa. In the Julian Calendar year -1 was directly followed by year 1, so there has not been a year 0. The extra day every four years was approximately 11 Minutes too long which accumulated to a discrepancy of one day each 128 years.
Gregorian Calendar
The Gregorian Calendar has been implemented on October 15, 1582 by pope Gregor XIII. To compensate the wrong leap days of the Julian Calendar, ten days were omitted in this year. October 4 has been followed by October 15. The Gregorian Calendar added 97 leap days every 400 years. In the years dividable by 100, leap days were omitted, except for years with centuries dividable by 4.
In non-catholic countries, the Gregorian Calendar was used later sometimes. In the UK in 1751 and in Russia in 1918 for example.
Easter Sunday and Holidays
Easter Sunday also was defined by the council of Nicäa. It is the first sunday after the first full moon in spring. Exceptions: April 19 always is chosen instead of April 26 and April 18 sometimes is chosen instead of April 25.
Following choice of other Holidays can be computed relative to Easter Sunday:
