The Romans sometimes neglected to introduce an extra month every two years to amortize the difference between their lunar calendar and the natural solar year. Julius Caesar decreed that the year 46 BC should have 445 days (some historians implausibly say: 443 days) in order to bridge the yawning discrepancy that accumulated over the preceding seven centuries. It was aptly titled the "Year of Confusion".
To "reset" the calendar, Julius Caesar affixed the New Year on January 1 (the day the Senate traditionally convened) and added a day or two to a few months.
He thus gave rise to the Julian Calendar, a latter day rendition of the Aristarchus calendar from 239 BC. After his assassination, the month of Quintilis was renamed Julius (July) in his honor.
The Julian calendar estimated the length of the natural solar year (the time it takes for the earth to make one orbit of the sun) to be 365 days and 6 hours. Every fourth year the extra six hours were collected and added as an extra day to the year, creating a leap year of 366 days.
But the calendar's underlying estimate was off by 11 minutes and 14 seconds. It was longer than the natural solar year. The extra minutes accumulated to one whole day. By 325 AD, the Spring Equinox was arriving on March 21st on the Julian Calendar - instead of March 25.
The First Ecumenical Council met in Nicea in 325 and determined that the date to celebrate Pascha was on the first Sunday, after the first full moon, after the Spring Equinox on March 21st. In other words, it enshrined the Julian calendar's aberration.
Thus, by 1582, the Spring Equinox was arriving on March 11. Half-hearted measures by Popes Paul III and Pius V failed to restore the essential correspondence between the calendar and the seasons.
Pope Gregory XIII decided - in his tenth year in office - to drop 3 leap years every 400 years by specifying that any year whose number ended with 00 must also be evenly divisible by 400 in order to have a 29-day February.
This would have the effect of bringing the Julian calendar closer to the natural length of the solar year - though an error of 26 seconds per year would still remain.
To calibrate the Julian calendar with the Gregorian one and to move the Spring Equinox back to March 21, 10 days were dropped from the civil calendar in October 1582. Thursday, October 4 was followed by Friday, October 15. People rioted in the streets throughout Europe, convinced that they have been robbed of 10 days.
But this was merely a convenient fiction. The Spring Equinox in the Gregorian calendar was, indeed, celebrated on March 21 in perpetuity. But, according to the Julian calendar, in the 17th century it arrived on March 11th, in the 18th century on March 10th, in the 19th century on March 9th, and in the 20th century on March 8th - 13 days earlier that even the erroneous date adopted by the Nicea Council.
The Gregorian calendar was controversial in Protestant countries. Britain and its colonies adopted it only in 1752. They had to drop 11 days from the civil calendar and move the official new year from March 25 to January 1. For centuries, dates followed by OS ("Old Style") were according to the Julian calendar and dates followed by NS ("New Style") according to the Gregorian one. Sweden adopted the Gregorian Calendar in 1753, Japan in 1873, Egypt in 1875, Eastern Europe between 1912 to 1919 and Turkey in 1927. In Russia it was decreed by the (bourgeois) revolutionaries that thirteen days would be omitted from the calendar, the day following January 31, 1918 becoming February 14, 1918.
It was Pope Pius X who, in 1910, changed the beginning of the ecclesiastical year from Christmas Day to January 1, effective from 1911 onwards.
All that time, the Christian Orthodox continued to observe the Julian calendar. In 1923, a Conference of Orthodox Churches in Constantinople reduced the number of leap years every 900 years and attained a discrepancy between the calendar and the natural solar year of merely 2.2 seconds per year.
According to this calendar, the Spring Equinox will regress by one day every 40,000 years.
They, too, had to drop 13 days to bring the Spring Equinox back to March 21st. Hence the gap between December 25 (Gregorian calendar) and January 7 (revised Julian-Orthodox calendar).
