A week [Latin - 'vicis' meaning 'change'] was originally an interval between Market Days, i.e. the day on which farmers and traders would gather at a particular location to buy and sell goods. The duration was five days in Central Asia, seven in Babylon, eight in Rome and ten days in Egypt. But slowly the seven-day week gained prominence and today it is used as a standard division of time around the world. A week as defined by Merriam-Webster's Online Dictionary is, 'any of a series of 7-day cycles used in various calendars; especially a 7-day cycle beginning on Sunday and ending on Saturday.'
In ancient time when man looked up towards the sky and saw the sun and the moon, the planets and the stars, he believed that the earth was the center of the universe and everything revolved around it. As the study progressed, man separated the different heavenly objects into stars, planets etc. Without the use of the instruments like telescopes, only a certain part of the sky was decipherable and it was thus assumed that our solar system consisted of the sun, the moon and five other planets. These were Mercury, Venus, Mars, Jupiter and Saturn. If you've realized, these are the planets that are either nearest to Earth or the biggest and thus visible with the naked eye.
The Egyptians believed that the seven objects were Gods and governed each hour of the day. The moon was thought to be the closest to Earth and was followed by Mercury, Venus, Sun, Mars, Jupiter and Saturn. The sequence in which they ruled the hours was considered to be in the inverse order of their distance from Earth. Thus the first hour was ruled by Saturn, the second by Jupiter, the third by Mars and so on. At the end of the seventh hour, the entire sequence was repeated, thus Saturn also ruled the 8th, 15th and 22nd hours, Jupiter managed the 23rd hour, Mars the 24th and the 1st hour of the next day was governed by the Sun.
Another belief was that the God that ruled the first hour of the day also ruled the day, thus the first day was ruled by Saturn, the second day by the Sun, the third by the moon, followed by Mars, Mercury, Jupiter and Venus.
It is also fascinating to note that the same solar system objects and in the same sequence have been used in ancient India, Tibet and Burma as also Japan to name the days of the weeks.
It is now easy to see how some of the days were named. For instance, Saturday is Saturn's day (a Roman God of planting and harvest), Sunday is sun's day, while Monday is moon's day (Anglo-Saxon monandaeg).
And as for the remaining...
Tuesday is derived from Mars, the Roman god of war. The Germanic (Teutonic) word for the warrior god is 'Tiu' or 'Tiw' and thus Tiw's-day.
Wednesday is Mercury's or Wotan's-day. 'Wotan' was a Teutonic God of war, who usurped many of the roles and functions of Tiu as he became more and more popular.
Thursday is Jupiter's-day. Jupiter was the Roman God of thunder and lightning. Thursday comes from the Germanic thunder-god Donar, who was known as 'Thor' to the Scandinavians. Thursday is thus Thor's-day or Torsdag.
Friday is the day of Venus, the Roman goddess of spring and the Greek goddess of love. The Scandinavian goddess of love is 'Frigg' from whose name is derived Friday or Frigg's-day.
But how is it that although according to the above, Saturday should be the first day of the week; we usually consider Sunday to be the first day?
According to one theory, the seventh day of the week in Hebrew is Shabbat or a day of holiness and rest, which is observed from sunset on Friday to nightfall on the next day, i.e. Saturday. And thus Saturday was the more important day, but its importance waned, as Christianity, for whom Sunday was a very important day, took deeper roots. Sunday in Christianity is the day on which God rested after creating the Universe, it is also the day on which Jesus Christ was resurrected and thus a holy day and a day of prayer and rest.
Another source states that with the increasing worship of the sun, Sunday, the day of the sun, gained in prominence and soon became the first day of the week.
Of course it is not necessarily always the first day of the week, there are still some countries, especially in Europe, that have Monday as the first day of the week. In fact the International Standard 'ISO 8601:1988 (E)' states that
5.4 Combinations of date and time of day representations
3.0 Terms and Definitions
3.17 Week Calendar
"A seven-day period within a calendar year, starting on Monday and identified by the ordinal number within a year..."
But whatever the theories, it is fascinating to know that each time someone asks you what day of the week it is, the answer resonates not just in your mind but also in time and space, touching a chord centuries back in history and on planets, as far away as Saturn.