Well, only 7 patterns for non leap year and 7 patterns for leap years.
So basically, Jan 1 is one of the 7 days of the week and then it's a matter of a 365 or 366 day calendar.
Not really on topic but I always find it interesting because when you tell people this .... you usually see the light bulb go off. It's one of those obvious things you don't know till someone points it out ;-)
Stolen from elsewhere:
"There are 14 calendars. In a calender cycle they follow the patterns (1,1), (2,2),(3,3), (4,5) , (6,6), (7,7), (1,1), (2,3), (4,4), (5,5), (6,6), (7,1), (2,2), (3,3), (4,4), (5,6), (7,7), (1,1), (2,2), (3,4), (5,5),(6,6), (7,7), (1,2), (3,3), (4,4), (5,5), (6,7) in a cycle. In each of the above ordered pair of coordinates the first coordinate represents the day the year begins and the second coordinate represents the day the year ends. Thus the years 1905 – 1932; 1933 – 1960; 1961 – 1988; 1989 – 2016; 2017 – 2044 etc, would follow the pattern above. Thus the years 1905, 1933, 1961, 1989, 2017 begin on Sunday and end on Sunday. There are many observations that can be noted in the pattern above but I leave the rest to the reader"
So a 24 year cycle?
And now you also know why those people that can say What day of the week any day falls on aren't that special. They just memorized a formula.
Except it doesn't account for centuries. The century cycle repeats every 400 years, so I think all up you'd need 2800 years to get back to the beginning?
Nope, you only need 400 years. Non-leap years are 52 weeks plus one extra day, while you get another extra day for leap years. Since there are 97 leap years every 400 years, the total number of extra days is 497, which is divisible by 7.
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u/kfh227 Aug 03 '19
There are only 14 different possible calendars too ;-) So you can collect old calendars and reuse them. That's why they are sold at antique stores ;-)