A week of seven days is natural if one divides a lunar month of
twenty-eight days into quarters. That is almost certainly the reason
we use seven. The weeks can start on recognizable phases of the moon,
new, quarter, full. Of course, a twenty-eight day month does not mesh
that well with a twelve month, 365 day year, so most calendars end up
being something of a mess of compromises and contradictions.
Twelve months, to be
sure, are a tidy way of dividing the year. If each were equal, mostly
likely at thirty days, there are only five days left over. Those can
be addressed in various ways, as outside the normal sequence of days
and weeks. A thirteen month year has its own advantages and
disadvantages; it does not line up with the solstices and equinoxes
in the same manner as twelve months.
That may or may not
be bothersome to a society. If we do use a thirty day month, the
weeks needn’t be of seven days. Ten or five might do as well, or
six, for that matter. Or, as long as we are not tying the months to
the moon, they might be of some other length altogether. Forty-five,
for example—half of a season. That could be divided neatly into
weeks of five or nine days.
All these are things
to be considered in world-building. They do assume a world with the
same length of year and phases of the moon as our own; both fantasy
and science fiction may have to deal with worlds with completely
different rules. The Exura where most of my fantasy fiction is set
does not present this difficulty, but I do keep in mind that other
worlds are not necessarily the same.
I have given my Mura
the thirteen month calendar. Yes, with seven day weeks. They do not
line up at all with the seasons, though the solstices and equinoxes
do always end up on the same date. Only one day is not named as a
normal day, belonging to neither a week nor a month, and that is the
New Year Day, celebrated at the spring equinox. On leap years, an
extra day is inserted just before it. These are the names of the
months, as use in the Muram Empire:
the Month of the Ram
— March 21 to April 17
the Month of the Fox — April 18 to
May 15
the Month of the Bull — May 16 to June 12
the
Month of the Rooster — June 13 to July 10
the Month of the
Grasshopper — July 11 to August 7
the Month of the Lion —
August 8 to September 4
the Month of the Fish — September 5 to
October 2
the Month of the Hawk — October 3 to October 30
the
Month of the Horse — October 31 to November 27
the Month of
the Wolf — Novemeber 28 to December 25
the Month of the Crow —
December 26 to January 22
the Month of the Rabbit — January 23
to February 19
the Month of the Frog — February 20 to March 19
New Year Day —
March 20
Most of the old Muram
kingdoms across the Greater Sea from the Empire utilize some variant
of this calendar, although the names of the months may differ. It was
never adopted, however, by the Sharshites or other nationalities
ruled by the Mura at some point. We can take a look at some of their
calendars another time.