Most often in May, 28 days.
Highest follower count seen that year: 55,398.
January
Jan 2 06:00–11:00 UTC Jan 3 07:00–15:00 UTC Jan 4 06:00–15:00 UTC Jan 10 06:00–14:00 UTC Jan 11 12:00–20:00 UTC Jan 12 06:00–11:00 UTC Jan 13 07:00–15:00 UTC Jan 14 06:00–13:00 UTC Jan 15 06:00–15:00 UTC Jan 16 06:00–14:00 UTC Jan 18 06:00–15:00 UTC Jan 19 06:00–12:00 UTC Jan 20 06:00–12:00 UTC Jan 22 07:00–12:00 UTC Jan 28 07:00–15:00 UTC Jan 29 07:00–15:00 UTC Jan 30 06:00–14:00 UTC
February
Feb 1 13:00–18:00 UTC Feb 2 06:00–14:00 UTC Feb 3 06:00–12:00 UTC Feb 4 06:00–12:00 UTC Feb 5 06:00–15:00 UTC Feb 6 06:00–14:00 UTC Feb 10 07:00–15:00 UTC Feb 12 07:00–13:00 UTC Feb 14 00:00–05:00 UTC Feb 15 06:00–12:00 UTC Feb 16 06:00–15:00 UTC Feb 17 06:00–14:00 UTC Feb 18 06:00–14:00 UTC Feb 19 06:00–11:00 UTC Feb 20 07:00–15:00 UTC Feb 21 09:00–14:00 UTC Feb 23 06:00–12:00 UTC Feb 24 08:00–15:00 UTC Feb 25 05:00–13:00 UTC Feb 26 06:00–14:00 UTC Feb 27 06:00–11:00 UTC
March
Mar 3 06:00–11:00 UTC Mar 4 06:00–14:00 UTC Mar 5 06:00–15:00 UTC Mar 6 05:00–12:00 UTC Mar 11 06:00–13:00 UTC Mar 12 05:00–11:00 UTC Mar 13 05:00–10:00 UTC Mar 14 06:00–15:00 UTC Mar 16 05:00–13:00 UTC Mar 17 05:00–14:00 UTC Mar 18 05:00–12:00 UTC Mar 19 06:00–14:00 UTC Mar 20 05:00–12:00 UTC Mar 21 06:00–15:00 UTC Mar 22 07:00–15:00 UTC Mar 23 07:00–15:00 UTC Mar 24 06:00–16:00 UTC Mar 25 07:00–15:00 UTC Mar 26 06:00–14:00 UTC Mar 28 06:00–13:00 UTC Mar 29 06:00–14:00 UTC Mar 30 07:00–13:00 UTC
April
Apr 3 06:00–13:00 UTC Apr 4 07:00–16:00 UTC Apr 7 05:00–11:00 UTC Apr 8 06:00–14:00 UTC Apr 9 05:00–11:00 UTC Apr 10 06:00–14:00 UTC Apr 11 06:00–14:00 UTC Apr 12 05:00–12:00 UTC Apr 15 05:00–12:00 UTC Apr 16 07:00–14:00, 18:00–24:00 UTC Apr 17 00:00–02:00, 06:00–11:00 UTC Apr 18 05:00–11:00 UTC Apr 19 09:00–17:00 UTC Apr 20 06:00–14:00 UTC Apr 21 07:00–14:00 UTC Apr 22 07:00–13:00 UTC Apr 23 05:00–13:00 UTC Apr 24 05:00–14:00 UTC Apr 26 06:00–12:00 UTC Apr 28 07:00–15:00 UTC Apr 29 06:00–14:00 UTC Apr 30 05:00–12:00 UTC
May
May 1 06:00–14:00 UTC May 2 05:00–12:00 UTC May 3 06:00–13:00 UTC May 4 05:00–14:00 UTC May 5 05:00–11:00 UTC May 6 06:00–14:00 UTC May 7 05:00–14:00 UTC May 8 07:00–14:00 UTC May 9 09:00–16:00 UTC May 10 08:00–14:00 UTC May 12 05:00–14:00 UTC May 13 01:00–06:00 UTC May 14 06:00–14:00 UTC May 15 05:00–13:00 UTC May 16 12:00–19:00 UTC May 17 07:00–14:00 UTC May 18 05:00–12:00 UTC May 19 07:00–15:00 UTC May 21 06:00–11:00 UTC May 22 09:00–14:00 UTC May 23 16:00–24:00 UTC May 24 14:00–22:00 UTC May 25 06:00–13:00 UTC May 27 07:00–14:00 UTC May 28 06:00–13:00 UTC May 29 07:00–14:00 UTC May 30 06:00–15:00 UTC May 31 06:00–11:00 UTC
June
Jun 1 06:00–14:00 UTC Jun 2 06:00–13:00 UTC Jun 3 06:00–13:00 UTC Jun 4 06:00–13:00 UTC Jun 5 06:00–14:00 UTC Jun 6 07:00–15:00 UTC Jun 7 07:00–13:00 UTC Jun 8 05:00–14:00 UTC Jun 9 06:00–12:00 UTC Jun 10 07:00–15:00 UTC Jun 13 05:00–13:00 UTC Jun 15 08:00–15:00 UTC Jun 17 07:00–14:00 UTC Jun 19 06:00–15:00 UTC Jun 22 08:00–12:00 UTC Jun 25 05:00–11:00 UTC Jun 26 05:00–13:00 UTC Jun 28 06:00–14:00 UTC Jun 30 15:00–21:00 UTC
July
Jul 1 20:00–21:00 UTC Jul 2 00:00–01:00 UTC Jul 3 06:00–11:00 UTC Jul 4 07:00–15:00 UTC Jul 5 07:00–15:00 UTC Jul 7 16:00–21:00 UTC Jul 8 22:00–24:00 UTC Jul 9 00:00–04:00, 20:00–24:00 UTC Jul 10 00:00–04:00, 20:00–24:00 UTC Jul 11 00:00–04:00, 07:00–14:00 UTC Jul 12 06:00–14:00 UTC Jul 13 12:00–19:00 UTC Jul 14 12:00–19:00 UTC Jul 15 12:00–15:00 UTC Jul 16 07:00–15:00 UTC Jul 17 07:00–12:00 UTC Jul 18 13:00–21:00 UTC Jul 19 13:00–19:00 UTC Jul 20 07:00–15:00 UTC Jul 21 07:00–15:00 UTC Jul 22 07:00–15:00, 23:00–24:00 UTC Jul 23 00:00–05:00, 13:00–20:00 UTC Jul 24 09:00–17:00 UTC Jul 25 08:00–14:00 UTC Jul 26 12:00–19:00 UTC