Maybe I’m the first mathematician who clearly defined the starting point (i.e. point Zero) for his calculations of the day of the week for Gregorian dates.
I'm using as starting point - Sunday, January, 0 1900.
Only for this date (consisting of Sunday, day 0 of month January, century 19xx and year xx00) ALL offsets (key numbers) have the value Zero. Even the Moon was not visible - Dark Moon!
The first day after this starting point was Monday, January 1st 1900, the first day in the first month of the first year in century 19, BTW a day of a New moon.
This conditions are defining clearly a set of parameters for day of the week, day, month, century, and year from 0 - 6.
If a parameter has a different value instead Zero this is because of it is
- an other day in the month
- an other month
- an other century
- an other year in century (considering leap days)
An different count of the days of the week (traditionell is Sunday the first day and often Sun=1 instead of Sun=0) or different key numbers for the century (20xx = 0 instead 19xx=0) leads us to an starting point with parameters different from Zero.
Es werden in sporadischer Folge mathematische und mnemotechnische Probleme und Methoden betrachtet, die es ermöglichen, den Wochentag für jedes Datum nach dem Gregorianischen oder Julianischen Kalender zu berechnen. Grundlage ist die "Enzyklopädie der Wochentagsberechnung" von Hans-Christian Solka, das umfassendste Werk zu diesem Thema.
