My Birthday’s on December 11th. Every year I notice that December 11th and December 25th are always the same day of the week. What mathematical things are happening to cause that?
My Birthday’s on December 11th. Every year I notice that December 11th and December 25th are always the same day of the week. What mathematical things are happening to cause that?
While this is the right answer, to nitpick, there is an exception: the Julian-to-Gregorian calendar switchover. It looks like the day-of-the-week system continued uninterrupted, but the day-of-the-month saw a jump.
https://en.wikipedia.org/wiki/Adoption_of_the_Gregorian_calendar
Where the switchover happened at the right time of the year, that same-day-of-the-week relationship won’t have been the case.
https://en.wikipedia.org/wiki/List_of_adoption_dates_of_the_Gregorian_calendar_by_country
So based on that table, a number of places did the switchover at some point between December 11th and December 25th, and for those that year, the relationship will not have held. In Styria, Austria, for example, the day after December 11th, 1583 was December 22nd, 1583.