The oldest calendar may be 30,000 years old. An engraved bone plaque found at Blanchard, in the Dordogne region of France, contains a series of 69 engravings arranged on a curved line. The shape of the engravings resembles the phases of the moon, and some archaeologists believe that that is what the marks represent. (source)

The Egyptian calendar, which was 365 days long and started on the day that Sirius rose in line with the sun, was instituted around 4,241 B.C. (source)

The year 46 B.C. was the longest year on record. By this year, the Roman calendar had fallen 80 days behind the seasons, so in order to make up for the accumulated slippage, Julius Caesar added two extra months to the year as well as 23 additional days in February. Thus, 46 B.C. was 455 days long.

The 13th of the month is more likely to fall on Friday than on any other day of the week. The 13th falls on Friday 688 times every 400 years, while it falls on Saturday and Thursday 684 times, Tuesday and Monday 685 times, and Wednesday and Sunday 687 times. (source)

There are some curiosities in the names for the months that the Romans gave us. July is named after Julius Caesar, and August after Augustus Caesar. September, October, November, and December come from the Latin words for seven, eight, nine, and ten, despite being the ninth through twelfth months. Originally the Romans had ten months, from March to December. Around 700 B.C., Numa Pompilius added the months of January and February. (source)

Months have different numbers of days, between 28 and 31, because of the Romans. Numa Pompilius assigned 29 days to seven months, 31 days to four months, and 28 days to just one month, because Romans thought that even numbers were bad luck. This totalled to 12 lunar months, which is just 355 days, so the calendar required frequent adjustments; later on various days were added to certain months. (source)

The origin of the Julian calendar dates to 46 B.C., when Julius Caesar, after adding 90 days to that year to make up for slippage in the calendar, decreed that each year thereafter that was divisible by 4 would be a leap year, with 366 days instead of the regular 365.

In 1654, Archbishop James Ussher of Armagh, having worked through the genealogy of the Bible, announced that the time of creation was on Sunday, October 21, 4004 B.C., at 9:00 in the morning. Ussher settled on the hour because it was a "civil" hour of the day and he figured God would be civil. (source)

Stonehenge may be a giant Neolithic calendar. The design of Stonehenge is such that, on the summer solstice (June 21), the rising sun is aligned with the avenue and perfectly bisects the stone circle. Stonehenge may have had other purposes, but whether it did or not is now a mystery.

The Lydians, who were allies of the Greek Spartans, and the Medes, who were dominated by Cyrus of Persia, had been locked in a five-year war in Asia Minor on May 28^th, 586 B.C., when the two armies were again preparing for another battle. At this point a solar eclipse happened, one that is believed to have been pre­dicted by Thales, a Greek mathematician. When the Medes and Lydians observed the eclipse, they stopped fighting and signed a peace treaty. Incidentally, this is the earliest event in human history to which an exact date can be assigned, due to the eclipse. (source)

Coin depicting Constantine.

Sunday first became a day of rest in the year 321. Roman emperor Constantine chose Sunday to please both Christians (the day of the resurrection) and pagans (many of whom worshipped one of the sun-gods of the empire).

No record exists of Christmas being celebrated on December 25^th before the year 336. (source)

In all likelihood, December 25^th is not the birthdate of Jesus. Most scholars believe that the date of December 25^th was chosen for Christmas because it coincided with both the winter solstice on the Julian calendar of the time and the birthdates of Mithras, the Persian sun-god, and Sol Invictis, another sun-god, and was near the pagan feasts of Saturnalia and the New Year. (source)

In the year 534, Dionysius Exiguus (also known as Dennis the Little), created the system, still used today, of counting the years starting with the birth of Christ. Unfortunately, he made some errors in calculation, so the birth of Jesus probably took place around 6 B.C. (Herod the Great, who is mentioned in the stories of Jesus' birth in the bible, died in 4 B.C.) (source)

There were two Thursdays one week in 1147. Pope Eugenius III travelled to Paris, and was scheduled to arrive on a Friday. In order that the Parisians could hold a celebration on Friday, a day of fast, Eugenius decreed that that day would be a Thursday. (source)

According to some interpretations of the Mayan "long count" linear calendar, the end of the world was to have happened in 2012.

The ancient Mayan calendar was more accurate than the modern Gregorian calendar. While the Gregorian calendar gains three days in 10,000 years, the Mayan calendar loses only two days every 10,000 years.

About 1250, the English scholar Roger Bacon (circa 1214–1292) noted that the year in the Julian calendar, then in use, was somewhat too long, as the vernal equinox came increasingly earlier each year. However, it took over 300 years, until 1582, until the corrective Gregorian calendar, the calendar now in use, was introduced. (source)

Pope Gregory XIII introduced the Gregorian Calendar in 1582. To correct the time error in the Julian calendar, which had been in use since 46 B.C., it was decreed that ten days (October 5–14, 1582) were to be omitted and it was ordained that, thereafter, years divisible by 100 but not divisible by 400 would not be leap years. Most Roman Catholic countries accepted these changes immediately. Protestant countries delayed for a while (for example, England waited until 1752). Other countries delayed even longer. For example, Greece didn't adopt the Gregorian calendar until 1912, and the last country to change over was Turkey, in 1927. (source)

Because the mathematician John Wallis was an extremely nationalistic Englishman, he used his influence against Great Britain's adoption of the Gregorian calendar. He argued that acceptance would imply subservience to Rome (and hence to foreigners). His view led to a long delay of the Gregorian calendar's adoption by Great Britain. (source)

In 1752, England adopted the Gregorian calendar. September 2, 1752 was followed immediately by September 14 because the Julian calendar then in use had become 11 days behind the seasons. When this happened, there was rioting in England, with crowds of people, believing that they had been deprived of eleven days of their lives, shouting "Give us back our eleven days!" (source)

George Washington, early military and political leader of the United States, was born, according to the Julian calendar in use at the time, on February 11. However, according to the Gregorian calendar, his birthday would be on February 22. In 1752, Great Britain and its colonies adopted the Gregorian calendar, so his birthday is given as February 22 in modern documents. The United States has a holiday to commemorate Washington's Birthday. It is celebrated on the third Monday in February, which always falls between February 15 and February 21 and so can never fall on either February 11 or February 22.

In the sixteenth century, there was no coherent way of dating events that had occurred in the distant past because of the many different calendars that were in use. To resolve this problem, Joseph Scaliger wrote A Treatise on the Correction of Chronology in which he proposed that events be dated by three different cycles: the 28 year solar cycle, the 19 year lunar cycle, and the 15 year period of Diocletian's tax census. Working backward, these cycles all started in 4713 B.C., which Scaliger numbered 1:1:1, and would repeat every 7980 years. Unfortunately, Scaliger's work was based on the Julian calendar, and a few months after his work was published, Pope Gregory XIII introduced the Gregorian calendar, rendering Scaliger's work useless. (source)

In 1929, the U.S.S.R. decreed a week of five days. In 1933, a six-day week was decreed. By 1940, the seven-day week was restored. (source)

Listopad means "October" in Croatian, and "November" in Czech. (source)

In English, the days of the week are named after the Saxon gods (except for Saturday, which is named after the Roman god of agriculture). Sunday is named after the sun, Monday after the moon, Tuesday after Tiw, Wednesday after Woden, Thursday after Thor, Friday after Frige, and Saturday after Saturn. (source)

On November 24, 1793, the National Convention in revolutionary France decreed a new "Revolutionary Calendar" with the aim of educating the public to new ideas such as eliminating wasteful holy days, such as Sundays and saints' days. It was similar to that used in ancient Egypt: Each year was divided into twelve months of thirty days each, with five extra days at the end of the year; each month had three ten-day "weeks." This calendar was mostly ignored by the end of the eighteenth century and was formally repealed by Napoleon in 1805, mainly because of the confusion caused by its abolition of the seven-day week. It had wreaked havoc with the traditional system of religious observances, festivals, and market days. (source)

The ancient Egyptians, and other ancients, defined the hour to be one-twelfth of the time between sunrise and sunset. So, as the days grew longer in winter and spring and shorter in summer and autumn, the length of the hour varied from one day to the next.

The third millennium and the 21^st century began on January 1^st, 2001, not January 1^st, 2000. When Dionysius Exiguus created the system of counting years starting with the birth of Christ, he did not include a year zero, as the concept of zero was not a familiar one to the Romans and Greeks. Therefore, the present calendar starts from the year 1, so the third millennium started in 2001. (source)

On years that are not leap years, if January 1^st falls on a Sunday, then the five months of January, April, July, October, and December of that year will contain a total of 25 Sundays. (source)