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 A178055 #11 Sep 04 2020 15:19:59
%S A178055 3,1,3,2,3,2,3,3,2,3,2,3,3,0,3,2,3,2,3,3,2,3,2,3,3,0,3,2,3,2,3,3,2,3,
%T A178055 2,3,3,0,3,2,3,2,3,3,2,3,2,3,3,1,3,2,3,2,3,3,2,3,2,3,3,0,3,2,3,2,3,3,
%U A178055 2,3,2,3,3,0,3,2,3,2,3,3,2,3,2,3,3,0,3,2,3,2,3,3,2,3,2,3,3,1,3,2,3,2,3,3,2
%N A178055 Numbers representing the number of days in a month in the Gregorian calendar (modulus 7).
%C A178055 Sequence first term represents January 2000. Sequence repeats after 4800 terms, representing 400 years in the Gregorian calendar system.
%C A178055 Actual number of days in a month can be determined by adding 28 to the value of the sequence term representing the month in question.
%H A178055 Lyle P. Blosser, <a href="/A178055/b178055.txt">Table of n, a(n) for n = 1..4800</a>
%H A178055 <a href="/index/Ca#calendar">Index entries for sequences related to calendars</a>
%e A178055 a(1) = 3 -> January 2000 has 31 days (3+28), a(2) = 1 -> February 2000 has 29 days (1+28), a(3) = 3 -> March 2000 has 31 days (3+28).
%t A178055 dys[{y_,m_,1}]:=If[m==12,DateDifference[{y,m,1},{y+1,1,1}],DateDifference[ {y,m,1},{y,m+1,1}]][[1]]; Mod[#,7]&/@(dys/@ Flatten[Table[{y,m,1},{y,2000,2010},{m,12}],1]) (* _Harvey P. Dale_, Sep 04 2020 *)
%Y A178055 Cf. A178054. If a(n) is the n-th term in A178054 and b(n) is the n-th term in A178055, then a(n) + b(n) (modulus 7) = a(n+1)
%K A178055 easy,nonn
%O A178055 1,1
%A A178055 Lyle P. Blosser (lyleblosser(AT)att.net), May 18 2010