<P> Calendars unrelated to the Chaldean, Hellenistic, Christian or Jewish traditions often have time cycles between the day and the month of varying lengths, sometimes also called "weeks". </P> <P> An eight - day week was used in Ancient Rome and possibly in the pre-Christian Celtic calendar . Traces of a nine - day week are found in Baltic languages and in Welsh . The ancient Chinese calendar had a ten - day week, as did the ancient Egyptian calendar (and, incidentally, the French Republican Calendar, dividing its 30 - day months into thirds). </P> <P> A six - day week is found in the Akan Calendar . Several cultures used a five - day week, including the 10th century Icelandic calendar, the Javanese calendar, and the traditional cycle of market days in Korea . The Igbo have a "market week" of four days . Evidence of a "three - day week" has been derived from the names of the days of the week in Guipuscoan Basque . </P> <P> The Aztecs and Mayas used the Mesoamerican calendars . The most important of these calendars divided a ritual cycle of 260 days (known as Tonalpohualli in Nahuatl and Tzolk'in in Yucatec Maya) into 20 weeks of 13 days (known in Spanish as trecenas). They also divided the solar year into 18 periods of 20 days and five nameless days, creating a 20 - day month divided into four five - day weeks . The end of each five - day week was a market day . </P>

Which roman emperor arranged the calendar into seven-day weeks