The calendar is a word that comes from Latin calendarium, or “account book”, and is derived from calendae or “the calends”, the first day of all of the old Roman months. This was the day on which accounts were due and on which the priests of Rome called the people together to proclaim (calare) that the new moon had been sighted. The earliest Roman calendar was a crude arrangement of ten lunar months, beginning with March (Martius), plus an undetermined long, unnamed period during the winter, when agriculture was moribund (in a dead period or inactive). Later, two new months, Januarius and Februarius, were created for this unnamed period and added to the end of the year. March continued as the first month of the year until 153 B.C., when the Roman state decreed that the new year would, from that time onward, begin on January 1. Intercalation of days was necessary from time to time, however, to realign this Roman calendar with the equinoxes. The word Intercalation refers to the insertion in the calendar of a day (or days) or a month (or months) to make the calendar year correspond to the solar year. Those whose duty it was to do the intercalations often abused their office by adding or subtracting days merely for political reasons to such an extent that by the time of Julius Caesar’s rule, the Roman calendar was thoroughly out of sync with January falling in autumn and the lengths of years highly unpredictable.
Julius Caesar is given credit for reforming the Roman calendar so it no longer relied on the lunar year, so then Caesar’s calendar, known as the Julian calendar, went into effect in 45 B.C. In the 16th century, a major overhaul of the Julian calendar was considered by both clergy and Christian governments who had strong incentives to provide a calendar by which Easter would be reckoned accurately. So the correct calculation of successive Easters was a vast challenge since it involved adjustments in computing both the solar and the lunar cycles used under the Julian calendar. The primary difficulty stemmed from an error in Sosigenes’ original reckoning of the length of the solar year: the Earth actually takes 11 minutes, 14 seconds less than 365.25 days to orbit the sun, in which this came from Egyptian astronomers who had calculated the solar cycle exactly enough to devise a yearly calendar of 365 days (12 months of 30 days each plus five extra days added as festival days at the end of the year). So as a result, the vernal equinox which marks the beginning of spring (which in Caesar’s time fell on or about March 25) did not hold stable in the Julian calendar but slipped backward over the centuries, until by the 16th century it was occurring on or about March 11.
So as early as the 13th century, proposals for calendar reform were made, and in the later Middle Ages the Roman papacy began to take the problem seriously, but further astronomical knowledge was necessary. With it being necessary, more problems resulted for the calendar makers because the Julian calendar imposed by the Roman Catholic Church was based on two suppositions: that the year contains 365.25 days and that 235 moons equaled 19 solar years, in which both were wrong. The Julian year was too long by about 11 minutes, and the error had accumulated to about 10 days by 1582. So to correct the Julian Calendar so it was in sequence with Easter and to avoid having to change all the missals and breviaries of the Roman Catholic Church, Pope Gregory XIII decreed that ten days be omitted from the calendar in 1582. So by this decree Thursday, October 4, was to be followed directly by Friday, October 15, in that year. Pope Gregory XIII then decreed that leap years would occur only when the year was divisible by four, and only the centennial years that were divisible by 400 would be leap years. During a leap year, one day is added to the month of February (the 29th), as a correction. This method of calendar keeping was gradually adopted across Europe, and the world, and is nearly universal today. The calendar reform finally promoted by Pope Gregory XIII on February 24, 1582, in his bull Inter Gravissimas rested on Nicolaus Copernicus’ new mathematical calculation of the motions of the heavenly bodies. The reform, especially regarding its lunar aspects, was the work of Aloysius Lilius, who was a physician at the University of Perugia, and, in its solar aspects, of the German Jesuit mathematician Christopher Clavius, who incorporated Lilius’ work into the calendar reform. The reformers also instituted January 1 as the beginning of the new year as in Roman times, but they did this for religious reasons, because the Gregorian calendar had to win acceptance in a Europe bitterly divided by the Protestant Reformation in which the changes were accepted almost immediately in lands whose rulers were Catholic.
Thenceforth the Julian calendar became known as the Old Style calendar (abbreviated O.S.) and the Gregorian as the New Style (N.S.). Because of religious differences (Catholic versus Protestants), the Gregorian reform that required the deletion of 11 days from the Julian calendar did not take place until 1752 when September 2 was followed directly by September 14 in the American colonies, Great Britain, and Ireland. Many people objected to the loss of those 12 days, among other reasons, because they felt that Pope Gregory’s changes meant that their lives were being shortened accordingly. As the Western European states became global imperial powers during the 19th and 20th centuries, countries around the world that had long followed their own ancient calendars moved to adopt the Gregorian style, at least for official business, often becoming two-calendar countries in the process. Most Muslim nations have compromised by employing both the Gregorian (New Style) and the Muslim calendars officially. Although many calendar revisions have been proposed in modern times, none have ever been accepted seriously by enough people to result in anything other than our current New Style system known as the Gregorian Calendar.
In the 4th century in 364, the Roman Council of Laodicea decided the doctrine of the Christian church and the contents of the Bible. It was at this Council that they outlawed the of keeping the Saturday sabbath, and in the process solemnifying Sunday as the Christian sabbath in (canon 29). So then all religious observances were to be conducted on Sunday, not Saturday. They ruled that Christians must not judaize by resting on the Sabbath, but must work on that day, rather honouring the Lord's Day; and, if they can, resting then as Christians, also decreeing that if any shall be found to be judaizers, let them be anathema from Christ. So the entire nation was then brought under the subjection of the Roman Catholic Church's religious observance of Sunday, this was one of the most fraudulent adjustment made the to the calendar, and remains today in 2009. Inquiries made in 1932 to the United States Naval Observatory, in Washington D.C., and the Royal Greenwich Observatory in London, England, have also confirmed that the weekly cycle of 7 days as observed today has not been altered, and remains as it has been since before the time of Christ. So both history and the bible make it quite clear that Sunday is the first day of the week and Saturday is indeed the seventh day, which has been kept by the Jews for millennia, even to the present day. The true seventh-day sabbath of God has not been lost, therefore our Saturday is the same day of the week today as the seventh-day sabbath of creation.