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            Re: Calendar reform dead as a practical issue?
            Fri, 13 Feb 1998 01:05:39 -0800
            Simon Cassidy <>

Amos quoted and answered the following question:

> > The calendar's wandering weekdays were designed with no consideration
> > at all given to the convenience of future calendar programmers. (How
> > easy is it for you to determine the weekday of January 1 from any given
> > year number?)
> Actually, it's not very difficult; you can even do it in your head!  See
> "the Doomsday method" [at].
> (IMHO, this algorithm is what motivated the originators of the Gregorian
> reform to choose it over the more accurate Persian 33-year cycle).

but Simon notes:

Even if we suppose that "this algorithm" was around in the 1580's (the
contemporary mathematician Conway is credited with its invention) it
would have not made weekday calculations any easier in pope Gregory's
calendar than in a calendar using the (Persian) 33-year cycle. In fact
"it" works a little better in my reconstruction of Dee's (or Dee and
Cecil's) calendar. I quote from the above web-site:

Dr. Conway now teaches the Doomsday algorithm, complete with Century
adjustment, using a very simple audiovisual aid -- your hand.
      ____/  ___)___    <---- Doomsday Difference
              ______)   <---- Century Day ("We-in-dis-day" for 1900)
              _______)  <---- number of DOZENS
      _____   ______)   <---- remainder
           \______)     <---- number of 4s in that remainder.

The Doomsday Difference is the difference between the required date and a
nearby Doomsday, recorded as so many days "on" (i.e. to be added) or "off"
(subtracted) from that Doomsday.

If the Dee-Cecil calendar had been adopted, A 17th. century "Conway"
would follow almost the same steps except for the "dozens" part:
      ____/  ___)___    <---- Doomsday Difference
              ______)   <---- Century Day ("We-in-Dee's-day" for 1600)
              _______)  <---- less number of 33s in century#+year#
      _____   ______)   <---- plus remainder
           \______)     <---- plus number of 4s in that remainder.

The Doomsday Difference (the difference between the required date and a
nearby Doomsday) being calculated in exactly the same fashion as Gregorian,
while the HARDEST piece of Conways method (getting other Century Days)
which is given at the above web-site as:

   Sun  Mon  Tue  Wed  Thu  Fri  Sat
  1700      1600 1500
  2100      2000 1900      1800
  2500      2400 2300      2200

What's the best way to memorize century Doomsdays? I'm not sure.
Here's what I use. Notice that century Doomsdays fall only on
"Sun-Tue-Wed-Fri". I say "Son to wed Friday", thinking of my own
son, and how pleased I would be if he were indeed getting married
(well, maybe not this Friday!). Combine this with Dr. Conway's
"We-in-dis-day" for 1900=Wednesday, and I can reconstruct the chart
mentally. The tricky part is that the years go right to left in
each row. The easy part is that if you can get just the one row
(the one with 1900=Wednesday in it), the other years are the same
Doomsday, plus or minus 400 years.

would be somewhat simpler in the Dee-Cecil calendar:
      Sun  Mon  Tue  Wed  Thu  Fri  Sat
                  1600           1500
   1700                1800
        1900                2000
             2100                2200
   2400                2500

i.e. add (or subtract) four weekdays for each century forward (or
from We-in-Dee's-day (WEDNESDAY) for the 1600s..

I show the above for the Dee-Cecil calendar rather than the original Dee
version only because the Dee-Cecil version is completely synchronous
the Gregorian calendar for the years 1585-1619 and the years 1981-2015.
The original Dee version would be 1 day ahead to get the VE on March 21
rather than March 20 (under the calendrical meridian at ~77 degrees W.).

available at
and (for those unfamiliar with my reconstruction of Dee's full proposal) see

Dee's Y'rs, Simon Cassidy, 1053 47th. St. Emeryville Ca. 94608.
ph.510-547-0684.                  email: