@Seneca: Natürlich gab es Kalender mit "Wochen" mit mehr als 7 Tagen (alte Ägypter: 10 Tage, Maya: 13/20 Tage, u.a.). In unserem jüdisch-christlichen Kalender dürfte es aber die Woche als 7-Tage-Einheit tatsächlich seit mehreren tausend Jahren (ununterbrochen!) geben:
The Christian, the Hebrew, the Islamic, and the Persian calendars all have a 7-day week.
Digging into the history of the 7-day week is a very complicated matter. Authorities have very different opinions about the history of the week, and they frequently present their speculations as if they were indisputable facts. In short, nothing can be said with certainty about the origin of the 7-day week.
The first pages of the Bible explain how God created the world in six days and rested on the seventh. This seventh day became the Jewish day of rest, the Sabbath, Saturday.
Extra-biblical locations sometimes mentioned as the birthplace of the 7-day week include: Babylon, Persia, and several others. The week was known in Rome before the advent of Christianity.
[...]
There is no record of the 7-day week cycle ever having been broken. Calendar changes and reform have never interrupted the 7-day cycles. It is very likely that the week cycles have run uninterrupted at least since the days of Moses (c. 1400 BC), possibly even longer.
http://www.tondering.dk/claus/cal/node8.html