Since ancient time people identified the planets in the sky, such as the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn, to the days of the week..
Sun has been assigned to Sunday, Moon to Monday, Mars to Tuesday, Mercury to Wednesday, Jupiter to Thursday, Venus to Friday and Saturn to Saturday.
The week originated in the pagan Mediterranean countries, and was embraced by Christianity and dispersed throughout Europe and beyond.
In English while Sunday Monday and Saturday relate to the Sun, Moon and Saturn, there is an Anglo-Saxon (Anglo-German) origin of the incorporation of the old Nordic (Germanic) god's names to the other days, such as Tiwes daeg for Tuesday, Wodnes daeg for Wednesday, Thunres daeg for Thursday and Frig Daeg for Friday.
However, the seven-day planet week comes to us from Rome. The celestial identities of the days are more evident in the Romance languages of Europe and these are related to Latin, which acknowledged the old planet gods with:
The Romans perpetuated and proliferated the seven-day planetary week, but it did not invent it. The Chaldaeans first engineered the week, with Mesopotamian planet gods, and introduced it to the Mediterranean countries in the Hellenistic (Greek) era, around 200 B.C.
Calendarical systems came into being since the beginning of civilizations. Ancients have undertaken to combine three natural measures of time — the solar day, the lunar month and the year of the seasons or the solar year. Calendars have therefore been of three types, lunar, lunisolar and solar.
The reckoning of the month by the Moon and the year by the Sun was widespread among the ancient civilizations. In India the concept of the month as a unit of time based on lunation developed in the Vedic times.
The Earth's revolution around the Sun, or the more obvious apparent motion of the Sun around the Earth, takes 365-1/4 days, the lunar months naturally became out of step with the seasons and with the solar year.
In order to make up the 11 days of difference it began during early periods by adding thirteenth month (intercalaray month) every 2-1/2 or 3 years to conform to the year of seasons.
Alternately even after making 12 months of 30 days each, it became necessary to add the difference of 5 to 6 days to the lunar year.
Subsequently the solar year of 365-1/4 days was adopted by distributing the additional days among the months by making them 30 or 31 days each and 28 days for February, which has an additional day every four years (leap year) to make up the shortfall of ¼ day in a year.
Published in The Hindu on Jan 10, 2002.