This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A101944 #13 May 03 2022 23:35:13
%S A101944 687,685,685,687,684,688,684
%N A101944 Number of months in a 400-year cycle that have the thirteenth day of the month fall on the n-th day of the week, beginning with Sunday.
%C A101944 Friday the thirteenth appears more often than any other day.
%C A101944 In the cycle of 400 Gregorian years there are 303 regular years with 365 days, and 97 leap years with 366 days, so the total number of days is 303*365 + 97*366 = 146097, which is divisible by 7. - _Amiram Eldar_, Apr 29 2022
%D A101944 Alfred S. Posamentier, Math Charmers, Tantalizing Tidbits for the Mind, Prometheus Books, NY, 2003, page 230.
%H A101944 E. G. Richards, <a href="http://www.users.zetnet.co.uk/egrichards/book.htm">Mapping Time, The Calendar and its History</a>, Oxford University Press, Great Clarendon Street, Oxford OX2 6DP, Reprinted 1999 (with corrections).
%H A101944 Wikipedia, <a href="https://en.wikipedia.org/wiki/Gregorian_calendar">Gregorian calendar</a>
%H A101944 <a href="/index/Ca#calendar">Index entries for sequences related to calendars</a>
%t A101944 (* first do *) Needs["Miscellaneous`Calendar`"] (* then *) t = Flatten[ Table[ DayOfWeek[{y, m, 13}], {y, 2000, 2399}, {m, 12}]]; d = Table[ DayOfWeek[{2006, 1, d}], {d, 1, 7}]; Table[ Count[t, d[[n]]], {n, 1, 7}]
%K A101944 fini,full,nonn,dumb
%O A101944 1,1
%A A101944 _Robert G. Wilson v_, Jan 28 2005