Subject:
Re: Dom.ltrs
Date:
Sat, 21 Dec 1996 15:37:14 -0800
From:
Simon Cassidy <simoncas@pacbell.net>
To:
CALNDR-L@ECUVM.CIS.ECU.EDU

I wrote:
>In general, the dominical letter of a date lets you know its weekday, once
>you also know the dominical letter OF THE YEAR, in the following manner:
>
>For any Gregorian year-number Y (normal or proleptic) the dominical letter
>of that year is derived from the following expression:
>
>       [Y + INT( Y/4 ) - INT( Y/100 ) + INT( Y/400 )] modulo 7
>
>where "INT( R )", of a real number R (e.g.13.9), is the integer part (e.g.13),
>and "[N] modulo 7" is the remainder when dividing N by 7 (casting out 7s).
>
>From this number (0,1,2,3,4,5 or 6) the dominical letter is derived thus:
>        0 = A,  1 = g,  2 = f,  3 = e,  4 = d,  5 = c,  6 = b.
>
>Given the dominical letter of a Gregorian year we know that all days with
>that dominical letter are Sundays.

However, I forgot to say that this does not apply to the beginning of leap
years when the dominical letter of the year is different from the letter
for the balance of the year (after leapday or "bissextus"). Thus the expression
above, for the dominical letter-number of the Gregorian year Y holds, in
general, only for March through December. For January and February use:

[Y + INT( (Y-1)/4 ) - INT( (Y-1)/100 ) + INT( (Y-1)/400 )] modulo 7

which will only be different from the simpler formula in Gregorian leap-years.
--
Dee's Y'rs, Simon Cassidy, 1053 47th.St. Emeryville Ca. 94608. ph.510-547-0684