International Calendar Traditions

The Gregorian calendar is the civil calendar of nearly the whole world. Introduced by Pope Gregory XIII in 1582, it keeps the...

The Badí' calendar of the Bahá'í Faith, created by the Báb and adopted by Bahá'u'lláh, has 19 months of 19 days plus four or five...

Moses Cotsworth proposed a perpetual calendar of 13 months of exactly 28 days, with a new month, Sol, between June and July and...

Elisabeth Achelis promoted the World Calendar from 1930: four identical quarters of 91 days with months of 31, 30 and 30 days...

The ISO week date in ISO 8601, first published in 1988, numbers weeks from Monday to Sunday. Week 1 is the week containing the...

The scientist Cesare Emiliani proposed in 1993 adding 10,000 to Common Era years, so 2026 becomes 12026 HE. The epoch roughly...

Unix time counts seconds since 00:00:00 UTC on 1 January 1970, ignoring leap seconds. It underlies timestamps in most computer...

Astronomers count days continuously as Julian Day Numbers from noon Universal Time on 1 January 4713 BCE in the proleptic Julian...

When the United States bought Alaska from Russia in 1867, the territory changed from the Julian calendar and from the Asian side...

January is named after Janus, the two-faced Roman god of doorways and beginnings. Rome's consuls took office on 1 January from...

February takes its name from februa, a Roman purification ritual held in the month. It was the last month of the earliest Roman...

March is named after Mars, the Roman god of war, and was the first month of the early Roman year. That origin explains why...

April comes from the Latin Aprilis, whose origin is uncertain; ancient writers linked it to aperire, to open, as of buds, while...

May is named after Maia, a Roman earth goddess associated with growth. May Day on the 1st combines old spring festivals with...

June is named after Juno, queen of the Roman gods and protector of marriage, which may underlie its popularity for weddings. The...

July was originally Quintilis, the fifth month of the early Roman year, and was renamed Iulius in 44 BCE to honour Julius Caesar...

August, formerly Sextilis, was renamed in 8 BCE for the emperor Augustus. The popular story that a day was taken from February to...

September comes from the Latin septem, seven, as it was the seventh month of the early Roman year. The September equinox, around...

October takes its name from octo, eight, its place in the early Roman year. It contains the most famous dates of calendar reform...

November is from novem, nine, the ninth month of the early Roman year. Several Roman emperors tried to rename months after...

December comes from decem, ten, as the tenth month of the early Roman year. It contains the December solstice, around 21 or 22...

Sunday is named after the Sun, following the Hellenistic planetary week, which assigned days to the seven classical planets...

Monday is the Moon's day, a translation of the Latin dies Lunae, which also gives French lundi and Spanish lunes. ISO 8601 makes...

Tuesday is named after Tiw, the Germanic sky and war god, who was equated with the Roman Mars of dies Martis, the source of...

Wednesday is Woden's day, after the chief Germanic god Woden or Odin, who was matched with Mercury, the Roman dies Mercurii and...

Thursday is Thor's day, the Germanic thunder god paired with Jupiter, as in Latin dies Iovis and French jeudi. In the ISO week...

Friday is named after the Germanic goddess Frigg or Freya, matched with Venus, as in Latin dies Veneris and French vendredi. It...

Saturday is the only English weekday that keeps a Roman name, from Saturn, dies Saturni. In many other languages its name comes...

A Gregorian year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 1600 and 2000...

Leap seconds are inserted into Coordinated Universal Time to keep it within 0.9 seconds of time based on the Earth's rotation...