Peter Vajk didn't mention the civil unrest and the unscrupulous behaviour
of landlords and merchants which occurred when some of these changeovers
were made. I use a somewhat reliable Calendar conversion application for
the Macintosh called "Intercal". Accompanying the application is a
treatise on calendrics written by the author of the application, Denis
Elliot, a Caltech graduate. I quote at length from his text following my
signature. This application is free and it and its very interesting pdf
addendum may be downloaded from the InfoMac archive.
Leigh
The Gregorian calendar was instituted by Pope Gregory XIII in 1582 A.D. It
had been noticed that the seasons were moving earlier and earlier compared
to given calendar dates. Equivalently, any fixed calendar date was
occurring later and later in the year as defined by the seasons. It was
clear that if the trend were allowed to continue, Easter (which is
supposed to be a spring time feast) would move into summer, and then
autumn, etc. The Pope dropped ten days so that the vernal equinox would
occur close to March 21. That was the date it had in the fourth century
A.D. at the time of the Council of Nicaea (325 A.D.). That Council had
developed the rule (which later became standard) used to compute the date
of Easter. The rule assumes that the vernal equinox is always on March 21.
By decree of the Pope, October 4 1582 was immediately followed by October
15 1582. The dates October 5 through October 14 1582 were simply dropped.
They did not exist. (This caused rioting in several cities. People thought
their lives were being shortened by 10 days.) The leap year rule was
changed to try to keep the calendar in synchronism with the seasons.
Finally, the rule for Easter was modified to keep it consistent with the
new leap year rule and the new epoch of the calendar.
Of course, the Pope is Catholic. So his decree was universally ignored
except in Roman Catholic countries and their colonies. However, religious
beliefs could not alter the fact that the seasons were noticeably sliding.
Gradually, Protestant countries adopted the Gregorian calendar. The dates
varied from country to country. The later a country made the change, the
more days had to be dropped. As European influence spread over the globe,
most non-Christian countries adopted the Gregorian calendar, at least for
civil use, as a matter of economic convenience. The last to make the
change were, in general, Eastern Orthodox Christians. Today this calendar
is the civil calendar nearly everywhere (but exceptions, especially in the
Middle East, do still exist). Sample conversion dates: England September 2
1752 was followed by September 14 1752; Japan in 1873; Turkey 1908; Greece
converted in 1923. When England switched, it also began reckoning the
start of a year as January 1.