Top 10 calendar facts

19. There was no year number zero. There were only 1999 years between 1000BC and 1000AD.

Tenth month. Originally, the Roman calendar was comprised only of 10 months, and December was the 10th and the last of them. Eventually a calendar reform introduced January and February, but preserved the ordinal naming convention of the older months.

Babylon. The seven-day week has deep historical origins in the astrology of Ancient Near East, being approximately a quarter of a lunation. It was (independently) adopted by the Persian Empire, in Judaism and in Hellenistic astrology, and (via Greek transmission) in Gupta India and Tang China.

31 Dec 2000. There was no year number zero. So, the first century started on 1 Jan 1. Similarly, 21th century started 1 Jan 2001.

24. The Gregorian Calendar omits 3 leap days every 400 years, omitting February 29 in the 3 century years (integer multiples of 100) that are not also integer multiples of 400. For example, 1600 was a leap year, but 1700, 1800 and 1900 were not. Similarly, 2000 was a leap year, but 2100, 2200, and 2300 will not be.

Pope Gregory XIII. Gregory XIII was pope during the period from 13 May 1572 until 10 April 1585. The Gregorian calendar, also called the Western Calendar, was introduced by him in 1582 with the Bull Inter Gravissimas. The Gregorian calendar basically a refinement of the Julian calendar. The Gregorian reform modified the Julian calendar's scheme of leap years, changing the mean length of the calendar year from 365.25 days to 365.2425 days.

Up to 3. Friday 13th happens three times in a year only if it starts on Thursday or (leap year) on Sunday.

This month starts on Sunday. If 1st is Sunday then Friday is on 6th and 13th.

7. January, March, May, July, August, October and December.

Blue moon. A blue moon is an additional full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month. Owing to the rarity of a blue moon, happens every two to three years, the term is used colloquially to mean a rare event, as in the phrase "once in a blue moon".