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?
Removed by mod
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
France adopted the new calendar with Sunday, 9 December 1582, being followed by Monday, 20 December 1582.
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.
Source?
It’s 14 days from Dec 11th to Dec 25th.
7 days after a Thursday, it’s a Thursday again. And then 7 days after that Thursday it’s once again a Thursday.
If Dec 25th was sometimes a few days later and sometimes a few days earlier (for example, if in some years the day after Dec 17th was Dec 21th and days 18, 19 and 20 would be skipped), then the weekdays would vary.
You can basically formulate your question in this way:
“If I look at what day is three days after a Tuesday, it’s always a Friday, regardless of year and week. Why is that?”
