"The origin of the seven-day week is the religious significance that was placed on the seventh day by ancient cultures, including the Babylonian and Jewish civilisations."

http://en.wikipedia.org/wiki/Seven-day_week

"Some Jewish, Christian, and Islamic groups have been historically opposed to the calendar because their tradition of worshiping every seventh day would result in either the day of the week of worship changing from year to year or eight days passing when Year Day or Leap Day occurs.[11] Others have contended that Year Day and Leap Day could be counted as additional days of worship."

http://en.wikipedia.org/wiki/International_Fixed_Calendar

"Thus, once or twice a year there would be eight days rather than seven between consecutive Saturdays. Thus the Jewish Sabbath, which must occur every seventh day, would be on a different weekday each year. The same applies to the Christian Sabbath. Hertz realised that this would cause problems for Jews and Christians alike in observing their Sabbaths, and mobilised worldwide religious opposition to defeat the proposal."

http://en.wikipedia.org/wiki/Joseph_Hertz#Calendar_reform