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 A309750 #12 Sep 13 2019 22:14:49
%S A309750 4,5,6,3,3,5,3,5,4,6,3,4
%N A309750 Number of letters in the English names of the months when the names are arranged in alphabetical order.
%C A309750 In alphabetical order, the months of the Gregorian calendar are: April (4), August (8), December (12), February (2), January (1), July (7), June (6), March (3), May (5), November (11), October (10), September (9).
%C A309750 Based originally on A260249 but with the additional layer of converting numbers to words and taking their lengths.
%F A309750 a(n) = A005589(A260249(n)).
%t A309750 Sort[Array[{DateValue[#, "MonthName"], StringLength@ IntegerName@ DateValue[#, "Month"]} &@ DateObject[{1, #, 1}] &, 12]][[All, -1]] (* _Michael De Vlieger_, Sep 04 2019 *)
%Y A309750 Cf. A005589, A260249.
%K A309750 nonn,fini,full,word
%O A309750 1,1
%A A309750 _Robert Dibley_, Aug 15 2019