Most often in March, 31 days.
Highest follower count seen that year: 40,006.
January
Jan 1 07:00–12:00 UTC Jan 2 06:00–12:00 UTC Jan 3 03:00–09:00 UTC Jan 4 05:00–11:00 UTC Jan 5 07:00–13:00 UTC Jan 6 07:00–12:00 UTC Jan 7 03:00–08:00 UTC Jan 9 04:00–06:00, 09:00–11:00 UTC Jan 10 06:00–11:00 UTC Jan 11 05:00–11:00 UTC Jan 12 07:00–12:00 UTC Jan 13 08:00–13:00 UTC Jan 14 03:00–13:00 UTC Jan 15 09:00–14:00 UTC Jan 16 07:00–13:00 UTC Jan 17 07:00–12:00 UTC Jan 18 04:00–07:00, 10:00–14:00 UTC Jan 19 08:00–14:00 UTC Jan 20 08:00–13:00 UTC Jan 21 04:00–09:00, 12:00–14:00 UTC Jan 22 08:00–13:00 UTC Jan 23 08:00–13:00 UTC Jan 24 04:00–11:00 UTC Jan 25 04:00–11:00 UTC Jan 26 07:00–12:00 UTC Jan 27 03:00–06:00 UTC Jan 28 08:00–12:00, 17:00–20:00 UTC Jan 29 11:00–13:00, 18:00–21:00 UTC Jan 30 11:00–13:00, 18:00–21:00 UTC
February
Feb 1 08:00–12:00 UTC Feb 2 02:00–07:00, 14:00–16:00 UTC Feb 3 07:00–11:00, 16:00–19:00 UTC Feb 4 07:00–11:00, 16:00–18:00 UTC Feb 5 10:00–14:00 UTC Feb 6 03:00–06:00, 09:00–13:00 UTC Feb 7 07:00–10:00, 14:00–16:00 UTC Feb 8 02:00–07:00 UTC Feb 9 02:00–07:00 UTC Feb 10 02:00–07:00, 10:00–13:00 UTC Feb 11 05:00–11:00 UTC Feb 12 01:00–06:00, 10:00–12:00 UTC Feb 13 02:00–04:00, 08:00–11:00 UTC Feb 14 00:00–07:00 UTC Feb 15 06:00–11:00 UTC Feb 16 05:00–11:00 UTC Feb 17 05:00–11:00 UTC Feb 18 03:00–05:00, 08:00–13:00 UTC Feb 19 02:00–05:00, 08:00–13:00 UTC Feb 20 02:00–05:00, 08:00–13:00 UTC Feb 21 05:00–10:00 UTC Feb 22 05:00–10:00 UTC Feb 23 09:00–14:00 UTC Feb 24 05:00–07:00, 11:00–16:00 UTC Feb 25 09:00–14:00 UTC Feb 26 04:00–06:00, 13:00–18:00 UTC Feb 27 04:00–07:00, 14:00–18:00 UTC Feb 28 05:00–10:00 UTC
March
Mar 1 07:00–09:00, 18:00–21:00 UTC Mar 2 05:00–08:00, 14:00–18:00 UTC Mar 3 06:00–08:00, 15:00–19:00 UTC Mar 4 14:00–17:00 UTC Mar 5 06:00–08:00, 15:00–19:00 UTC Mar 6 07:00–08:00, 16:00–18:00 UTC Mar 7 06:00–08:00 UTC Mar 8 07:00–09:00, 15:00–19:00 UTC Mar 9 05:00–07:00, 13:00–18:00 UTC Mar 10 05:00–08:00, 14:00–19:00 UTC Mar 11 06:00–09:00, 15:00–17:00 UTC Mar 12 05:00–08:00, 14:00–18:00 UTC Mar 13 05:00–08:00, 11:00–16:00 UTC Mar 14 05:00–08:00, 17:00–21:00 UTC Mar 15 06:00–08:00, 10:00–12:00 UTC Mar 16 05:00–08:00, 11:00–15:00 UTC Mar 17 04:00–06:00, 12:00–16:00 UTC Mar 18 04:00–06:00, 12:00–15:00 UTC Mar 19 01:00–06:00, 09:00–11:00 UTC Mar 20 01:00–05:00, 10:00–13:00 UTC Mar 21 10:00–13:00 UTC Mar 22 02:00–06:00, 11:00–13:00 UTC Mar 23 02:00–05:00, 10:00–12:00 UTC Mar 24 00:00–04:00 UTC Mar 25 02:00–07:00 UTC Mar 26 02:00–07:00, 14:00–16:00 UTC Mar 27 02:00–07:00 UTC Mar 28 02:00–08:00 UTC Mar 29 08:00–12:00 UTC Mar 30 02:00–06:00 UTC Mar 31 02:00–06:00, 12:00–15:00 UTC
April
Apr 1 01:00–06:00 UTC Apr 2 02:00–07:00 UTC Apr 3 01:00–05:00, 12:00–15:00 UTC Apr 4 01:00–06:00 UTC Apr 5 03:00–08:00 UTC Apr 6 02:00–07:00 UTC Apr 7 02:00–06:00 UTC Apr 8 01:00–07:00 UTC Apr 9 02:00–07:00, 11:00–14:00 UTC Apr 10 01:00–06:00 UTC Apr 11 04:00–07:00, 12:00–14:00 UTC Apr 12 02:00–07:00 UTC Apr 13 02:00–07:00 UTC Apr 14 01:00–06:00 UTC Apr 15 03:00–06:00, 08:00–10:00, 23:00–24:00 UTC Apr 16 00:00–04:00 UTC Apr 17 02:00–07:00 UTC Apr 18 03:00–08:00 UTC Apr 19 03:00–08:00 UTC Apr 20 02:00–07:00, 22:00–24:00 UTC Apr 21 00:00–04:00 UTC Apr 22 01:00–06:00 UTC Apr 23 01:00–06:00 UTC Apr 24 00:00–04:00 UTC Apr 25 05:00–09:00 UTC Apr 26 02:00–07:00 UTC Apr 27 10:00–12:00 UTC Apr 28 02:00–07:00 UTC Apr 29 01:00–06:00, 09:00–12:00 UTC Apr 30 02:00–05:00, 09:00–14:00 UTC
May
May 1 01:00–05:00 UTC May 2 01:00–07:00 UTC May 3 02:00–07:00 UTC May 4 05:00–06:00 UTC May 5 02:00–06:00, 11:00–14:00 UTC May 6 01:00–05:00, 10:00–13:00 UTC May 7 03:00–05:00, 10:00–14:00 UTC May 8 02:00–03:00, 05:00–06:00, 11:00–13:00 UTC May 9 03:00–08:00 UTC May 10 04:00–06:00 UTC May 11 02:00–04:00, 10:00–13:00 UTC May 12 02:00–06:00 UTC May 13 02:00–06:00, 10:00–13:00 UTC May 14 03:00–06:00, 10:00–14:00 UTC May 15 02:00–06:00, 09:00–11:00 UTC May 16 03:00–07:00 UTC May 17 04:00–07:00, 23:00–24:00 UTC May 18 00:00–03:00, 10:00–14:00 UTC May 19 02:00–05:00, 11:00–15:00 UTC May 20 04:00–06:00, 08:00–12:00 UTC May 21 03:00–05:00, 09:00–13:00 UTC May 22 03:00–05:00, 09:00–12:00 UTC May 23 03:00–10:00 UTC May 24 03:00–06:00, 14:00–17:00 UTC May 25 03:00–06:00 UTC May 26 04:00–07:00, 10:00–14:00 UTC May 27 03:00–06:00, 09:00–14:00 UTC May 28 03:00–06:00, 09:00–13:00 UTC May 29 03:00–06:00, 12:00–15:00 UTC May 30 04:00–09:00 UTC May 31 04:00–10:00 UTC
June
Jun 1 00:00–02:00, 10:00–14:00, 23:00–24:00 UTC Jun 2 00:00–03:00, 11:00–14:00 UTC Jun 3 01:00–05:00 UTC Jun 4 02:00–07:00, 10:00–13:00 UTC Jun 5 02:00–04:00, 11:00–13:00 UTC Jun 6 04:00–08:00 UTC Jun 7 02:00–06:00, 11:00–13:00 UTC Jun 8 02:00–04:00, 10:00–11:00 UTC Jun 9 01:00–06:00 UTC Jun 10 02:00–06:00 UTC Jun 11 02:00–06:00, 10:00–14:00 UTC Jun 12 02:00–04:00, 08:00–12:00 UTC Jun 13 03:00–10:00 UTC Jun 15 03:00–05:00, 09:00–15:00 UTC Jun 16 01:00–05:00, 09:00–16:00 UTC Jun 17 08:00–16:00 UTC Jun 18 04:00–09:00 UTC Jun 19 04:00–07:00 UTC Jun 20 04:00–09:00 UTC Jun 21 03:00–09:00 UTC Jun 22 04:00–06:00, 11:00–16:00 UTC Jun 23 04:00–05:00, 10:00–14:00 UTC Jun 24 03:00–08:00 UTC Jun 25 04:00–06:00, 10:00–14:00 UTC Jun 26 05:00–10:00 UTC Jun 27 05:00–11:00 UTC Jun 28 04:00–08:00 UTC Jun 29 04:00–08:00, 12:00–16:00 UTC Jun 30 00:00–03:00, 13:00–15:00 UTC
July
Jul 1 05:00–07:00, 12:00–15:00 UTC Jul 2 04:00–07:00, 11:00–16:00 UTC Jul 3 04:00–08:00 UTC Jul 4 04:00–09:00 UTC Jul 5 04:00–09:00, 14:00–17:00 UTC Jul 6 03:00–08:00, 15:00–17:00 UTC Jul 7 04:00–07:00, 13:00–16:00 UTC Jul 8 04:00–06:00, 08:00–14:00 UTC Jul 9 04:00–08:00 UTC Jul 10 03:00–06:00, 10:00–14:00 UTC Jul 11 05:00–10:00 UTC Jul 12 03:00–08:00 UTC Jul 13 03:00–06:00, 10:00–12:00 UTC Jul 14 02:00–05:00, 07:00–12:00 UTC Jul 15 02:00–03:00, 09:00–13:00 UTC Jul 16 03:00–05:00, 09:00–14:00 UTC Jul 17 02:00–06:00, 10:00–13:00 UTC Jul 18 04:00–07:00, 10:00–13:00 UTC Jul 19 02:00–06:00, 10:00–12:00 UTC Jul 20 02:00–07:00, 11:00–15:00 UTC Jul 21 03:00–06:00, 10:00–14:00 UTC Jul 22 02:00–07:00, 10:00–13:00 UTC Jul 23 04:00–08:00, 12:00–17:00 UTC Jul 24 05:00–06:00, 10:00–15:00 UTC Jul 25 04:00–07:00 UTC Jul 26 05:00–06:00, 11:00–13:00 UTC Jul 27 09:00–14:00 UTC