Months, weeks and days

This account is based on the (unpublished) book "Revolution in Time:The case for a New Calendar" by Robert Wolfe, formerly Professor of History at NYU.  Although the length of a year is determined by astrological (or astronomical) facts (in other words the movements of the sun and stars relative to the earth), the phases of the moon usually determine the months, and these are not exact in number of days, but usually vary from 29 to 31 days. 

There were ten months in the original Roman year, with some extra months added when necessary (Intercalans).  But then two months were added at the beginning of the year around the 5th century bce, when the Roman monarchy became the Roman Republic, named after the Roman God Janus and the month of ritual purification Februa, so that March, named after the Roman God of War Mars, was deliberately relegated for political reasons from first place to third.  So the months were then dedicated as follows: January (Janus), February (Februa), March (Mars), April (Aphrodite), May (Maia), June (Juno), July (Julius; originally Quintilis or fifth), August (Augustus; originally Sextilis or sixth), September (seventh), October (eighth), November (ninth), December (tenth).  Note that even though September-December are the ninth to twelth months, paradoxically their original names of the seventh to tenth months was retained. This was the form adopted in the Julian calendar.

In principle, there is no reason why the week should consist of 7 days.  In other calendars the equivalent of the week was 10 or even 20 days long. The Jewish religion was the first use 7 days and to make one day of the week, the seventh day, a Holy day of rest or Shabbat (Sabbath), the other days were and are simply named in Hebrew as first day (yom rishon), second (yom shani); and so on.  Originally the Roman month had three significant days, the Calens (first day), from which we get our word calendar, the Nones (the 9th day) and the Ides (the15th day).  But, with the transition from the Roman Republic to the Empire, under the influence of Jewish and Egyptian-Greek culture, the week was divided into seven days, but each day of the week was named according to the seven heavenly bodies that they could then see, namely Sunday (Sun), Monday (Moon), Tuesday (Tyr, Norse equivalent to Mars); Wednesday (Woden, Norse equivalent to Mercury), Thursday (Thor, Norse equivalent to Jupiter); Friday (Freya, Norse equivalent of Venus); Saturday (Saturn).

Thus weeks of 7 days were adopted by many calendars, but the Christian and Muslim religions that derived from Judaism, chose deliberately to make their "sacred" days different from that of Judaism, namely Sunday (also named Lord's Day) and Friday, respectively.