# Periodicity of calendar

Let $A$ be the set of days in week, $A = \{Mon,Tue,Wed,Thu,Fri,Sat,Sun\}$, and $B = \{1,2,\dots,31\}$. For each date, we have a pair $(x,y)$, $x \in A$ and $y \in B$. Then we will have an infinite sequence of these pairs. For example, starting from today (Friday, April 27, 2012), we have $(Fri,27), (Sat,28), (Sun,29),\dots$ and so on. Now my questions are:

1. What is the smallest period $T$ after which the sequence repeats? I know that in every 400 years there are 146097 days, which is a multiple of 7, therefore 146097 is a period. But does there exist a smaller period?
2. During each period $T$, what is (are) the pair (pairs) that appear the most (i.e. highest frequency)? What is the frequency of $(Fri,13)$?

Edit: we are using the Gregorian calendar, with different lengths of months, leap years, etc.

• Are we working on an actual calender, where we have different lengths of months, leap years, etc? Or do you want to know how long it takes to get a repeat if you just consider months of length 31? – Alex Becker Apr 28 '12 at 1:08
• There is no smaller period than $400$, because of the special rule about leap year in years divisible by $100$. – André Nicolas Apr 28 '12 at 1:13
• @AlexBecker: I've just edited the questions to address your comment. – Truong Apr 28 '12 at 1:14
• This table should answer the second part of the question: http://www.merlyn.demon.co.uk/freq-tbl.txt. – Doug Chatham Apr 28 '12 at 1:21

## 1 Answer

The table at http://www.merlyn.demon.co.uk/freq-tbl.txt gives the frequencies for each the days of the year over the minimum period, which is 400 years. Several days share the maximum frequency, including Friday the 13th (688/146097).

• The link is no longer valid. – Alexander Belopolsky Oct 6 '15 at 20:14