On Feb 19, 5:14*am, Ruud Harmsen wrote:
Fri, 19 Feb 2010 10:34:10 +0100: James Hogg :
in sci.lang:

What could be simpler?

The Jewish calendar.

the Jewish Calendar has a complicated algorithm, IIRC refined by the
famous 18th cent. mathematician Euler.
the complications are due to making sure that certain holidays do not
fall on certain days.

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



Hebrew calendar

....

Special holiday rules
Adjustments are made to ensure certain holy days and festivals do or
do not fall on certain days of the week.

Yom Kippur
Adjustments are made to ensure that Yom Kippur, on which no work can
be done, does not fall on Friday (the day prior to the Sabbath) to
avoid having Yom Kippur's restrictions still going on at the start of
Sabbath, or on Sunday (the day after Shabbat) to avoid having the
Shabbat restrictions still going on at the start of Yom Kippur.

The Rosh Hashanah postponement rules are the mechanism used to make
the adjustments. As Yom Kippur falls on Tishrei 10, and Rosh Hashanah
falls on the 1st, the adjustment is made so that Rosh Hashanah does
not fall on a Wednesday or Friday.



To ensure that Yom Kippur does not directly precede or follow Shabbat,
and that Hoshana Rabbah is not on a Shabbat, in which case certain
ceremonies would be lost for a year, the first day of Rosh Hashanah
may only occur on Mondays, Tuesdays, Thursdays, and Saturdays (the
"four gates"). Adjustments are made to ensure that Rosh Hashanah does
not fall on the other three days. To achieve that result the year may
be made into a short (chaser) year (both Kislev and Cheshvan have 29
days) or full (maleh) year (both Kislev and Cheshvan have 30 days).
(see table)

The day of the week on which Rosh Hashanah falls in any given year
will also be the day on which Sukkot and Shmini Atzeret will occur.




--
Ruud Harmsen,http://rudhar.com