King Canute may have failed to stop the rising of the tide when he commanded the sea to halt, but Pope Gregory XIII was able to decree the annulment of time itself; or to be more specific, he declared the erasure of 10 days in October of 1582, and he pulled it off in such a way that most of the world eventually accepted their disappearance (though not without some grumbling).
How was this possible?
As any student of the Talmud knows, it is the legal order that determines the calendar, not precise astronomical observation, and not correct mathematical calculation. An example: A famous rabbinic legend recorded in Tractate Rosh Hashanah of the Babylonian Talmud (Rosh Hashanah, 25a) recounts an argument between two sages – Rabbi Joshua ben Hananiah and his student, Rabban Gamaliel – over the correct date to celebrate the most solemn observance of the year, the Day of Atonement. As it happened, Rabban Gamaliel had the clear advantage in the debate, since his position was also the majority position of the highest court of the Jewish people, the Sanhedrin, which made it the law of the land. Rabbi Joshua, however, would not concede the point and refused to acknowledge the legitimacy of the Sanhedrins decision. Rabban Gamaliel decided that he needed to make an object lesson out of his teachers recalcitrance. Accordingly he forced Rabbi Joshua to come calling at Javneh, which was the seat of the Sanhedrin. He was to do this with his staff and purse in hand on the very day that should have been The Day of Atonement according to Rabbi Joshuas reckoning. But to travel and to carry a purse and staff on the Day of Atonement would be to profane the holiest day of the year! This was exactly Rabban Gamaliels point. It is the decision of the court, and not the individual sages personal reckoning, that establishes the proper times for holy observances.
So it was when the Council of Trent (1545-1563) met to work out the Roman Catholic Churchs official reaction to the Protestant Reformation, among other major crises of the day, that the subject of the calendar came up. Easter, the most important observance of the Christian year, was originally tied to the Jewish festival of Passover, which occurs in the beginning of the Spring. But because of deficiencies in the Julian Calendar which the church had followed since antiquity, the observance of Easter had begun to drift further and further away from the Spring equinox. Many held this to be a disastrous outcome. It was decided, therefore, on December 4, 1563, the last day of the Council of Trent, that the Pope should oversee a reform of the Missal and the Breviary, which included a requirement to reform the calendar (the calendar of observances was one part of the Breviary).
This was the genesis of the Gregorian Calendar. There were several suggestions circulating in the sixteenth century about how best to address the problems of the old calendar, but in 1572, the first year of his papacy, Gregory XIII gave leadership over the project to a Jesuit astronomer named Christopher Clavius (1537-1612). The method that Clavius would finally adopt followed calculations performed by Aloysius Lilius (d. 1576) a native of Calabria, and a professor of medicine at the University of Perugia. Lilius had spent ten years of his life working out the system for an improved calendar, but he never published his work in his lifetime. His brother, Antonius, submitted the plan in manuscript to the papal curia in 1576. That manuscript has been lost and no copy of Liliuss writing remains, but its contents are known from the publications of Clavius and his commission. Principle among these are Romani calendarii a Gregorio XIII restituti explicatio (Rome, 1603), and Novi calendarii Romani apologia (adversus M. Maestlinum in Tübingensi Academia mathematicum (Rome, 1588), both of which can be found in the Vatican Library.
Gregory promulgated the recommendations that Clavius commission made in a papal bull which has traditionally been called Inter Gravissimas. The name derives from the first two words of the text (an abbreviation of the phrase Inter gravissimas curas (lit. Among our gravest concerns). The bull indicated three objectives. First, the equinox had to be restored to March 21. March 21 was the date the fathers of the church at the Council of Nicaea identified as the equinox in 325AD when they established the date on which Easter should fall. Second, the date of Easter must be fixed once again to correspond with the first full moon following the vernal equinox, which would bring it closer in line with the Jewish Passover. To achieve this, a careful recalculation of lunar phases against the solar calendar had to be undertaken. Third, a system must be implemented whereby the discrepancies in the Julian calendar might be remedied so that the equinox would always fall on or about March 21. These objectives were to be met by creating a new set of lunar tables, implementing a reduction in the number of leap years (the Julian calendar had too many, which was one of the reasons for its tendency to drift), and last but not least, ten days in October of the year of the bulls announcement would be erased from the calendar. (Gregory specifies that for tax purposes and for loans, the ten missing days were not to be counted).
Gregorys immediate legal authority was limited to the Church and to the Papal States, but he urged all Christian princes to adopt his calendar. Naturally, the remaining Catholic kings of Europe were prompt in doing so, implementing the change over the course of the year 1582. The story was otherwise, however, in the Protestant countries, most of which would not adopt the Gregorian calendar until the eighteenth century. The English speaking world, for example, finally accepted Gregorys magical deletion of ten days with the passage at Parliament of what is known as the Calendar (New Style) Act of 1750. This law introduced the Gregorian calendar to England and all its colonies, including those that would become the United States of America. Incidentally, by the time the English set about reforming their calendar, the ten days specified by Gregory were no longer sufficient to bring the equinox back to March 21; to achieve the desired result, it was determined that October of 1752 must be shortened by 11 days. For that matter, the year 1751 had to be reduced to a shocking 282 days to bring England in line with the European standard. But that is another story.