The only reason for time is so that everything doesn't happen at once. Albert Einstein.
THE JULIAN/GREGORIAN CALENDAR CHANGE
The Julian calendar, introduced by the Romans and used in Western Europe for more than 1,500 years, consisted of 365.25 days with every fourth year being 366 days. This was a little too long since the correct value for the tropical year is 365.242199 days. This error of 11 minutes 14 seconds per year amounted to almost one and a half days in two centuries, and seven days in 1,000 years. The problem with this inaccuracy was placed before church councils, but no action was taken because the astronomers of the period did not have enough precise information concerning the exact value of the tropical year.
By 1545, however, the vernal equinox, which was used in determining Easter, had moved 10 days from its proper date. Pope Paul III was authorized to take action to correct the error but neither the Pope nor his immediate successors where able to find a satisfactory solution.
When Pope Gregory XIII was elected in 1572, he found various proposals awaiting him and agreed to issue a bull that the Jesuit astronomer Christopher Clavius (1537-1612) began to draw up, using suggestions made by the astronomer and physician Luigi Lilio.
The papal bull appeared in March 1582 and in order to bring the vernal equinox back to March 21, several arrangements had to be made. First, the day following the Feast of St. Francis (i.e. October 5) was to become October 15, thus omitting 10 days. Second, to bring the year closer to the true tropical year, a value of 365.2422 days was accepted. This value differed by 0.0078 days per year from the Julian calendar reckoning, amounting to 0.78 days per century, or 3.12 days every 400 years. It was therefore stated that there would be one leap year every four years except when the year is a centennial (1700, 1800, 1900). But the year 2000 would be a leap year since it is a multiple or 400.
The papal bull also established the rules for calculating the date of Easter, which was the most important feast of the Christian Church, and its place in the calendar determined the position of the rest of the church's movable feasts.
Because its location on the calendar depends on both the Moon's phases and the vernal equinox, clerical authorities had to seek some way of reconciling lunar and solar calendars. This system had to be simple so that even the common people would be able to predict its occurrence. There was no easy or obvious solution, and to make things more difficult there was no unanimous agreement on the way in which Easter should be calculated, even on a lunar calendar.
With these in mind, the problem could be broken down into two parts: first, devising a simple but effective way of calculating the days of the week for any date in the year and, second, determining the date of the Full Moons in any year. The first part was solved by the use of a letter code derived from a similar Roman system adopted for determining market days. For ecclesiastical use, the code gave what was known as the Sunday, or dominical, letter.
Because the Gregorian calendar used a more accurate value for the tropical year than the Julian calendar and achieved this by omitting most centennial leap years, Clavius decided that, when the cycle of epacts reached an ordinary centennial year, the number of the epact should be reduced by one; this reduction became known as the solar correction.
The obvious advantages of the Gregorian calendar were not accepted everywhere because of the breakdown between Eastern and Western Christianity and of Protestants and Roman Catholics. Other places were extremely slow in adapting to the new calendar. In France, Italy, Luxembourg, Portugal, and Spain, the New Style calendar was adopted in 1582, and it was in use by most of the German Roman Catholic states as well as by Belgium and part of the Netherlands by 1584. Switzerland's change was gradual, on the other hand, beginning in 1583 and being completed only in 1812. Hungary adopted the New Style in 1587, and then there was a pause of more than a century before the first Protestant countries made the transition from the Old Style calendar. In 1699-1700, Denmark and the Dutch and German Protestant states embraced the New Style, although the Germans declined to adopt the rules laid down for determining Easter. The Germans preferred to rely instead on astronomical tables and specified the use of the Tabulae Rudolphinae (Rudolphine Tables), based on the 16th-century observations of Tycho Brahe. They acceded to the Gregorian calendar rules for Easter only in 1776. Britain adopted the New Style in 1752 and Sweden in 1844. Japan adopted the New Style in 1873; Egypt adopted it in 1875; and between 1912 and 1917 it was accepted by Albania, Bulgaria, China, Estonia, Latvia, Lithuania, Romania, and Turkey. The now-defunct Soviet Union adopted the New Style in 1918, and Greece in 1923.
The Alaskan territory retained the Old Style calendar until 1867, when it was transferred from Russia to the United States.
In Britain and the British dominions, the change was made when the difference between the New and Old Style calendars amounted to 11 days: the British Calendar Act of 1751 made the day following Wednesday 2nd September 1752 into Thursday 14th September, omitting 11 days. Also, the first day of the year was moved from 25th March to the first of January (e.g. 24th March 1700 was followed by 25th March 1701). To stop the financial world from losing 11 days interest, the start of the financial year was moved to the 6th April. Some date records may show dual years, e.g. 25th February 1750/51, as the date could be in either year depending on which calendar was used.
1751 therefore became the shortest year as it consisted only of 25 March through to 31 December & so was only 282 days long.
There was widespread misunderstanding among the public, however, even though legislation authorizing the change had been framed to avoid injustice and financial hardship.
The following is an extract of the British Calendar Act of 1751:
C A P. XXIII.
A.D. 1751. Anno vicesimo quarto GEORGII II.
An Act for Regulating the Commencement of the Year; and for Correcting the Calendar now in Use.
The old Supputation of the Year not to be made use of after Dec. 1751. Year to commence, for the future, on 1 Jan.
That in and throughout all his Majesty's Dominions and Countries in Europe, Asia, Africa, and America, belonging or subject to the Crown of Great Britain, the said Supputation, according to which the Year of our Lord beginneth on the 25th Day of March, shall not be made use of from and after the last Day of December 1751; and that the first Day of January next following the said last Day of December shall be reckoned, taken, deemed and accounted to be the first Day of the Year of our Lord 1752; and the first Day of January, which shall happen next after the said first Day of January 1752, shall be reckoned, taken, deemed and accounted to be the first Day of the Year of our Lord 1753; and so on, from Time to Time, the first Day of January in every Year, which shall happen in Time to come, shall be reckoned, taken, deemed and accounted to be the first Day of the Year; and that each new Year shall accordingly commence, and begin to be reckoned, from the first Day of every such Month of January next preceding the 25th Day of March, on which such Year would, according to the present Supputation, have begun or commenced:
The Days to be numbered as now until 2 Sept. 1752;
and the Day following to be accounted 14 Sept. omitting 11 Days.
And that from and after the said first Day of January 1752, the several Days of each Month shall go on, and be reckoned and numbered in the same Order; and the Feast of Easter, and other moveable Feasts thereon depending, shall be ascertained according to the same Method, as they now are, until the 2nd Day of September in the said Year 1752 inclusive; and that the natural Day next immediately following the said 2nd Day of September, shall be called, reckoned and accounted to be the 14th Day of September, omitting for that Time only the 11 intermediate nominal Days of the common Calendar; and that the several natural Days, which shall follow and succeed next after the said 14th Day of September, shall be respectively called, reckoned and numbered forwards in numerical Order from the said 14th Day of September, according to the Order and Succession of Days now used in the present Calendar; and that all Acts, Deeds, Writings, Notes and other Instruments of what Nature or Kind soever, whether Ecclesiastical or Civil, Publick or Private, which shall be made, executed or signed, upon or after the said first Day of January 1752, shall bear Date according to the said new Method of Supputation,
The British & European Gregorian Perpetual Calendars can be used to show these changes to the calendar.