Zeller's Birthday
The Day-of-Week is in essence the remainder obtained by dividing
the Day-Count by seven :-
1. Day-Count rises with Day-of-Month
2. For March-based years, Zeller observed that Day-Count rises
near-linearly with Month, and the length of February becomes
irrelevant,
3. For March-based years, Day-Count rises by 365 per year, with
correcting terms for each applicable Leap Year Rule depending only
on the year
4. If only Day-of-Week is needed, multiples of seven can be omitted
throughout.
For Rule 2, Zeller used a factor of 26/10, equivalent to 30.6 days
per month, presumably the simplest to work with. It appears that
367/12 = 28 + 31/12 = 30.583 days per month, which might be
considered more logical*, can also be used.
* : If February were an "ordinary short" month, the year would have
367 days.
http://www.merlyn.demon.co.uk/zeller-c.htm#Prs