Is there a name or technical term for the instance when two dates always fall on the same day of the week? (Ex. June 13th and September 19th always share the same day of the week, this year being a Sunday.)
Also, how many of these instances exist?
Much thanks in advance!
I don’t know of a word specifically for your question, but it is a type of synchronicity.
I think what you’ve found is pretty trivial so there is no name for it. Take any two dates on the same day of the week. If the interval don’t span leap day then they will always fall on the same day of the week as each other. I guess the answer is really 365[sup]2[/sup] depending on how strictly you define it.
Whoops, my mistake. For any given date there are nominally 51 other dates that fall on the same day of the week. Correct answer is much smaller, less than 365*51 if you exclude duplicate combinations.
I’ve never seen it applied to the calendar, but there’s a concept in math that might apply.
If you converted the dates Jan 1-Feb 28 to consecutive counting numbers then:
1 is congruent to 8 modulo 7 because both 1 and 8 leave 1 as a remainder when divided by 7.
The rest of the calendar couuld be done separetely Apr 1-Dec 31.