Most often in January, 30 days.
Highest follower count seen that year: 5,044.
January
Jan 1 08:00–12:00 UTC Jan 2 04:00–12:00 UTC Jan 3 05:00–08:00, 13:00–14:00 UTC Jan 4 07:00–12:00 UTC Jan 5 06:00–12:00 UTC Jan 6 05:00–07:00, 11:00–14:00 UTC Jan 7 05:00–09:00 UTC Jan 8 06:00–07:00, 11:00–13:00 UTC Jan 9 06:00–08:00 UTC Jan 10 05:00–09:00 UTC Jan 11 05:00–11:00 UTC Jan 12 05:00–10:00 UTC Jan 13 02:00–06:00 UTC Jan 14 05:00–11:00 UTC Jan 15 04:00–14:00 UTC Jan 16 04:00–12:00 UTC Jan 17 05:00–13:00 UTC Jan 18 04:00–09:00 UTC Jan 19 05:00–12:00 UTC Jan 20 04:00–09:00, 11:00–15:00 UTC Jan 21 06:00–14:00 UTC Jan 22 04:00–09:00, 11:00–14:00 UTC Jan 23 04:00–09:00 UTC Jan 24 06:00–10:00 UTC Jan 25 06:00–09:00, 11:00–12:00 UTC Jan 27 05:00–09:00 UTC Jan 28 07:00–08:00, 10:00–14:00 UTC Jan 29 05:00–07:00, 12:00–13:00 UTC Jan 30 05:00–10:00 UTC Jan 31 05:00–13:00 UTC
February
Feb 1 06:00–14:00 UTC Feb 2 05:00–14:00 UTC Feb 3 05:00–13:00 UTC Feb 4 08:00–09:00 UTC Feb 6 04:00–12:00 UTC Feb 7 05:00–11:00 UTC Feb 8 06:00–09:00 UTC Feb 9 04:00–09:00 UTC Feb 10 06:00–09:00 UTC Feb 11 04:00–09:00 UTC Feb 12 05:00–07:00 UTC Feb 13 10:00–12:00 UTC Feb 16 09:00–13:00 UTC Feb 17 04:00–10:00 UTC Feb 18 05:00–06:00, 09:00–10:00 UTC Feb 19 04:00–08:00 UTC Feb 21 05:00–14:00 UTC Feb 22 06:00–10:00 UTC Feb 23 05:00–12:00 UTC Feb 24 05:00–09:00 UTC Feb 25 04:00–09:00, 11:00–12:00 UTC Feb 26 05:00–09:00 UTC Feb 27 06:00–11:00 UTC Feb 28 06:00–12:00 UTC
March
Mar 1 06:00–14:00 UTC Mar 2 05:00–13:00 UTC Mar 3 09:00–13:00 UTC Mar 5 05:00–09:00 UTC Mar 6 09:00–10:00, 12:00–14:00 UTC Mar 7 06:00–12:00 UTC Mar 8 05:00–09:00 UTC Mar 9 05:00–09:00 UTC Mar 10 05:00–07:00 UTC Mar 11 05:00–11:00 UTC Mar 12 04:00–11:00 UTC Mar 13 04:00–07:00, 09:00–10:00 UTC Mar 14 04:00–11:00 UTC Mar 15 04:00–12:00 UTC Mar 16 04:00–12:00 UTC Mar 17 04:00–09:00 UTC Mar 18 05:00–13:00 UTC Mar 19 05:00–12:00 UTC Mar 20 06:00–07:00 UTC Mar 21 05:00–13:00 UTC Mar 22 04:00–12:00 UTC Mar 23 05:00–12:00 UTC Mar 24 04:00–11:00 UTC Mar 25 04:00–12:00 UTC Mar 26 04:00–12:00, 17:00–21:00 UTC Mar 28 05:00–12:00 UTC Mar 29 05:00–12:00 UTC Mar 30 05:00–12:00 UTC Mar 31 06:00–13:00 UTC
April
Apr 2 04:00–12:00 UTC Apr 3 04:00–10:00 UTC Apr 4 04:00–09:00 UTC Apr 6 05:00–12:00 UTC Apr 7 04:00–11:00 UTC Apr 9 05:00–11:00 UTC Apr 10 05:00–12:00 UTC Apr 11 04:00–12:00 UTC Apr 12 04:00–09:00 UTC Apr 14 05:00–11:00 UTC Apr 15 05:00–12:00 UTC Apr 16 04:00–11:00 UTC Apr 17 04:00–07:00, 10:00–12:00 UTC Apr 18 05:00–10:00 UTC Apr 19 05:00–12:00 UTC Apr 20 04:00–12:00 UTC Apr 21 05:00–07:00, 09:00–13:00 UTC Apr 22 05:00–12:00 UTC Apr 23 05:00–11:00 UTC Apr 24 05:00–13:00 UTC Apr 25 05:00–13:00 UTC Apr 26 05:00–12:00 UTC Apr 27 05:00–13:00, 17:00–19:00 UTC Apr 28 05:00–09:00 UTC Apr 29 05:00–13:00 UTC Apr 30 05:00–12:00 UTC
May
May 2 05:00–07:00 UTC May 3 06:00–13:00 UTC May 4 06:00–12:00 UTC May 5 05:00–13:00 UTC May 6 06:00–12:00 UTC May 7 06:00–13:00 UTC May 8 05:00–13:00 UTC May 9 05:00–13:00 UTC May 10 06:00–10:00 UTC May 11 05:00–11:00 UTC May 12 04:00–11:00 UTC May 13 08:00–10:00 UTC May 14 05:00–10:00 UTC May 15 05:00–12:00 UTC May 16 04:00–12:00 UTC May 17 05:00–11:00 UTC May 18 04:00–10:00 UTC May 19 04:00–12:00 UTC May 20 04:00–12:00 UTC May 21 07:00–09:00 UTC May 22 05:00–12:00 UTC May 23 05:00–11:00 UTC May 24 05:00–11:00 UTC May 25 05:00–12:00 UTC May 26 09:00–13:00 UTC May 27 05:00–11:00 UTC May 29 05:00–12:00 UTC May 30 05:00–10:00 UTC May 31 05:00–11:00 UTC
June
Jun 1 05:00–12:00 UTC Jun 2 05:00–11:00 UTC Jun 5 09:00–13:00 UTC Jun 6 05:00–11:00 UTC Jun 7 06:00–12:00 UTC Jun 8 05:00–10:00 UTC Jun 9 05:00–08:00 UTC Jun 10 05:00–08:00 UTC Jun 11 06:00–11:00 UTC Jun 12 05:00–08:00 UTC Jun 13 06:00–08:00 UTC Jun 15 04:00–09:00 UTC Jun 16 04:00–08:00 UTC Jun 17 10:00–12:00 UTC Jun 18 04:00–05:00 UTC Jun 20 09:00–10:00 UTC Jun 21 08:00–11:00 UTC Jun 22 05:00–09:00 UTC Jun 23 06:00–08:00, 10:00–11:00 UTC Jun 24 05:00–11:00 UTC Jun 25 05:00–13:00 UTC Jun 26 05:00–08:00 UTC Jun 27 05:00–08:00 UTC Jun 28 06:00–08:00, 10:00–12:00 UTC Jun 29 06:00–12:00 UTC Jun 30 05:00–10:00 UTC
July
Jul 1 07:00–12:00 UTC Jul 2 04:00–12:00 UTC Jul 3 05:00–12:00 UTC Jul 4 05:00–12:00 UTC Jul 5 06:00–12:00 UTC Jul 6 05:00–09:00 UTC Jul 7 05:00–11:00 UTC Jul 8 05:00–12:00 UTC Jul 9 06:00–13:00 UTC Jul 10 05:00–12:00 UTC Jul 11 06:00–13:00 UTC Jul 12 04:00–11:00 UTC Jul 13 05:00–08:00, 10:00–12:00 UTC Jul 14 08:00–12:00 UTC Jul 15 08:00–09:00, 11:00–13:00 UTC Jul 16 04:00–06:00, 08:00–09:00, 11:00–13:00 UTC Jul 17 04:00–13:00 UTC Jul 18 07:00–13:00 UTC Jul 19 05:00–11:00 UTC