There are 97 leap days every 400 years, then the calendar repeats. So you have 303/400 chance of not having a leap year, and in those years, you get a 1/7 chance of having this calendar. Thus 303/2800.
This is counterintuitive to me, because 303/2800 is .108, which is between 1/9 and 1/10. But 97 out of 400 is less than 1 out of 4, so it shouldn’t be able to interfere more than twice in a 7 year cycle, on average. But your math looks correct. I must be missing something.
I think it’s more like 303/2800 chance.
There are 97 leap days every 400 years, then the calendar repeats. So you have 303/400 chance of not having a leap year, and in those years, you get a 1/7 chance of having this calendar. Thus 303/2800.
This is counterintuitive to me, because 303/2800 is .108, which is between 1/9 and 1/10. But 97 out of 400 is less than 1 out of 4, so it shouldn’t be able to interfere more than twice in a 7 year cycle, on average. But your math looks correct. I must be missing something.