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https→—on the first day, the first visible crescent appeared; on approximately the seventh, the waxing half-moon could be seen; on approximately the fourteenth, the full moon; on approximately the twenty-first, the waning half-moon; and on approximately the twenty-eighth, the last visible crescent. As you can see, each notable cycle is made up of about seven days, hence, the seven-day week.
Want the text version?: http://www.todayifoundout.com/index.php/2013/04/the-origin-of-the-7-day-week-and-the-names-of-the-days-of-the-week/
Sources:
http://www.crowl.org/lawrence/time/days.html
http://www.speakfrench.co.uk/vocab/days_of_the_week
http://library.thinkquest.org/4626/days.htm
http://www.timeanddate.com/calendar/days/
http://www.studyspanish.com/lessons/days.htm
http://www.pantheon.org/articles/f/freya.html
http://en.wikipedia.org/wiki/Seven-day_week
http://www.etymonline.com/index.php?term=sabbath
http://en.wikipedia.org/wiki/Seven-day_week
http://www.hermetic.ch/cal_stud/hlwc/why_seven.htm
http://en.wikipedia.org/wiki/Julian_calendar
http://en.wikipedia.org/wiki/Nundinal_cycle#Nundinal_cycle
http://en.wikipedia.org/wiki/Names_of_the_days_of_the_week
