Most often in January, 31 days.
Highest follower count seen that year: 14,180.
January
Jan 1 14:00–15:00 UTC Jan 2 01:00–12:00 UTC Jan 3 02:00–12:00, 14:00–15:00 UTC Jan 4 04:00–15:00 UTC Jan 5 02:00–12:00, 14:00–16:00 UTC Jan 6 02:00–05:00, 07:00–12:00 UTC Jan 7 03:00–05:00, 07:00–16:00, 21:00–24:00 UTC Jan 8 00:00–03:00, 06:00–12:00 UTC Jan 9 01:00–12:00 UTC Jan 10 00:00–04:00, 07:00–15:00 UTC Jan 11 03:00–05:00, 11:00–13:00, 15:00–24:00 UTC Jan 12 00:00–02:00, 07:00–09:00, 14:00–17:00 UTC Jan 13 03:00–05:00, 07:00–12:00, 14:00–16:00 UTC Jan 14 03:00–05:00, 08:00–12:00, 14:00–19:00 UTC Jan 15 02:00–05:00, 07:00–12:00 UTC Jan 16 00:00–12:00 UTC Jan 17 00:00–12:00, 14:00–17:00 UTC Jan 18 01:00–12:00 UTC Jan 19 02:00–13:00 UTC Jan 20 02:00–20:00 UTC Jan 21 04:00–12:00, 15:00–18:00 UTC Jan 22 04:00–13:00 UTC Jan 23 03:00–11:00 UTC Jan 24 05:00–06:00, 08:00–13:00, 15:00–16:00 UTC Jan 25 04:00–05:00, 07:00–15:00, 22:00–23:00 UTC Jan 26 00:00–05:00, 07:00–10:00, 16:00–18:00 UTC Jan 27 04:00–17:00 UTC Jan 28 02:00–05:00, 08:00–12:00 UTC Jan 29 02:00–08:00, 11:00–13:00, 16:00–19:00 UTC Jan 30 03:00–21:00 UTC Jan 31 02:00–16:00, 23:00–24:00 UTC
February
Feb 1 00:00–01:00, 03:00–12:00, 14:00–16:00 UTC Feb 2 03:00–04:00, 06:00–12:00 UTC Feb 3 02:00–04:00, 06:00–12:00, 17:00–19:00 UTC Feb 4 10:00–13:00, 15:00–21:00 UTC Feb 5 10:00–13:00 UTC Feb 6 04:00–09:00, 11:00–13:00 UTC Feb 7 02:00–15:00 UTC Feb 8 01:00–15:00 UTC Feb 9 02:00–05:00, 11:00–13:00 UTC Feb 10 00:00–02:00, 04:00–13:00 UTC Feb 11 01:00–07:00, 09:00–12:00 UTC Feb 12 01:00–12:00, 15:00–16:00 UTC Feb 13 04:00–12:00 UTC Feb 14 01:00–12:00, 14:00–16:00 UTC Feb 15 01:00–05:00, 07:00–15:00 UTC Feb 16 01:00–04:00, 06:00–12:00 UTC Feb 17 01:00–13:00 UTC Feb 18 01:00–04:00 UTC Feb 19 01:00–04:00, 06:00–08:00 UTC Feb 20 02:00–12:00 UTC Feb 21 04:00–05:00, 07:00–11:00, 14:00–15:00 UTC Feb 22 01:00–05:00, 07:00–15:00 UTC Feb 23 01:00–05:00, 07:00–09:00 UTC Feb 24 00:00–12:00 UTC Feb 25 02:00–05:00, 07:00–12:00 UTC Feb 26 01:00–12:00 UTC Feb 27 01:00–05:00, 07:00–12:00 UTC Feb 28 02:00–05:00, 10:00–11:00 UTC
March
Mar 1 02:00–06:00, 08:00–13:00, 15:00–16:00 UTC Mar 2 00:00–05:00, 07:00–12:00 UTC Mar 3 01:00–04:00, 07:00–12:00 UTC Mar 4 01:00–12:00 UTC Mar 5 01:00–13:00 UTC Mar 6 01:00–04:00, 09:00–13:00 UTC Mar 7 02:00–04:00, 07:00–16:00 UTC Mar 8 01:00–04:00, 08:00–12:00, 14:00–15:00 UTC Mar 9 00:00–05:00, 08:00–12:00 UTC Mar 10 01:00–08:00, 12:00–13:00 UTC Mar 11 01:00–04:00, 06:00–12:00 UTC Mar 12 00:00–04:00, 07:00–12:00 UTC Mar 13 00:00–05:00, 07:00–12:00 UTC Mar 14 00:00–05:00, 07:00–13:00 UTC Mar 15 01:00–04:00, 06:00–14:00, 22:00–24:00 UTC Mar 16 01:00–04:00, 06:00–11:00 UTC Mar 17 00:00–09:00 UTC Mar 18 01:00–05:00, 07:00–12:00 UTC Mar 19 00:00–05:00, 07:00–13:00 UTC Mar 20 00:00–09:00 UTC Mar 21 01:00–05:00 UTC Mar 22 00:00–12:00 UTC Mar 23 03:00–04:00, 06:00–08:00, 11:00–12:00, 15:00–21:00 UTC Mar 24 03:00–12:00 UTC Mar 25 06:00–12:00 UTC Mar 26 04:00–12:00 UTC Mar 27 01:00–12:00 UTC Mar 28 01:00–09:00 UTC Mar 29 00:00–04:00, 07:00–14:00 UTC Mar 30 01:00–04:00, 06:00–08:00, 11:00–12:00 UTC Mar 31 01:00–12:00 UTC
April
Apr 1 00:00–12:00 UTC Apr 2 01:00–12:00 UTC Apr 3 00:00–13:00 UTC Apr 4 02:00–14:00 UTC Apr 5 01:00–05:00, 07:00–13:00 UTC Apr 6 00:00–12:00 UTC Apr 7 00:00–01:00, 03:00–04:00, 07:00–12:00 UTC Apr 8 00:00–05:00, 09:00–11:00 UTC Apr 10 09:00–11:00 UTC Apr 11 03:00–13:00 UTC Apr 12 00:00–12:00 UTC Apr 13 00:00–13:00, 23:00–24:00 UTC Apr 14 00:00–08:00, 10:00–13:00, 23:00–24:00 UTC Apr 15 00:00–13:00, 16:00–17:00 UTC Apr 16 01:00–04:00, 08:00–12:00, 20:00–22:00 UTC Apr 17 02:00–05:00, 10:00–13:00 UTC Apr 18 00:00–13:00 UTC Apr 19 00:00–02:00, 06:00–12:00 UTC Apr 20 00:00–05:00, 07:00–12:00 UTC Apr 21 01:00–08:00, 17:00–19:00 UTC Apr 22 04:00–05:00, 07:00–11:00, 14:00–17:00 UTC Apr 23 04:00–05:00, 08:00–13:00 UTC Apr 24 02:00–03:00, 08:00–12:00 UTC Apr 25 00:00–04:00, 06:00–13:00 UTC Apr 26 00:00–04:00, 06:00–14:00 UTC Apr 27 01:00–12:00 UTC Apr 28 00:00–05:00, 07:00–13:00 UTC Apr 29 00:00–04:00, 11:00–13:00 UTC Apr 30 03:00–12:00 UTC
May
May 1 00:00–04:00 UTC May 2 03:00–04:00 UTC May 3 01:00–13:00, 15:00–16:00 UTC May 4 01:00–02:00, 07:00–13:00 UTC May 5 00:00–05:00, 07:00–08:00, 10:00–12:00 UTC May 6 01:00–04:00, 07:00–12:00, 19:00–20:00 UTC May 7 04:00–13:00, 15:00–17:00 UTC May 8 08:00–12:00, 17:00–20:00 UTC May 9 09:00–11:00, 14:00–20:00 UTC May 10 02:00–11:00 UTC May 11 01:00–10:00, 14:00–24:00 UTC May 12 11:00–12:00 UTC May 13 01:00–04:00, 07:00–13:00 UTC May 14 01:00–04:00, 06:00–12:00 UTC May 15 03:00–12:00 UTC May 16 01:00–02:00, 04:00–15:00 UTC May 17 00:00–03:00, 07:00–12:00 UTC May 18 00:00–04:00, 07:00–13:00 UTC May 19 00:00–05:00 UTC May 20 01:00–04:00, 10:00–13:00 UTC May 21 01:00–04:00, 07:00–12:00 UTC May 22 00:00–12:00 UTC May 23 00:00–05:00, 09:00–12:00 UTC May 24 01:00–14:00 UTC May 25 00:00–13:00 UTC May 26 01:00–04:00, 07:00–13:00, 18:00–22:00 UTC May 27 02:00–09:00, 11:00–12:00 UTC May 28 03:00–04:00, 07:00–12:00 UTC May 29 02:00–05:00, 11:00–13:00 UTC May 30 01:00–05:00, 09:00–12:00, 14:00–15:00 UTC May 31 00:00–04:00, 08:00–10:00, 12:00–14:00 UTC
June
Jun 1 00:00–04:00, 06:00–12:00 UTC Jun 2 01:00–06:00, 10:00–12:00 UTC Jun 3 00:00–03:00 UTC Jun 4 00:00–05:00, 08:00–12:00 UTC Jun 5 01:00–05:00, 09:00–11:00 UTC Jun 6 00:00–05:00 UTC Jun 7 03:00–06:00, 08:00–13:00 UTC Jun 9 03:00–05:00 UTC Jun 10 01:00–05:00, 07:00–10:00, 17:00–18:00 UTC Jun 11 00:00–04:00, 07:00–12:00 UTC Jun 12 01:00–05:00, 08:00–13:00 UTC Jun 13 04:00–06:00, 09:00–13:00 UTC Jun 15 02:00–04:00, 06:00–12:00 UTC Jun 16 00:00–05:00, 07:00–12:00 UTC Jun 17 05:00–09:00 UTC Jun 18 02:00–04:00, 06:00–13:00 UTC Jun 19 01:00–06:00, 18:00–19:00 UTC Jun 20 03:00–12:00 UTC Jun 21 03:00–05:00, 07:00–13:00 UTC Jun 22 02:00–12:00 UTC Jun 23 00:00–01:00, 05:00–12:00 UTC Jun 24 02:00–12:00 UTC Jun 25 03:00–12:00 UTC Jun 26 01:00–12:00 UTC Jun 27 00:00–14:00 UTC Jun 28 01:00–05:00, 07:00–13:00 UTC Jun 29 00:00–12:00 UTC Jun 30 00:00–04:00, 06:00–12:00, 16:00–17:00, 23:00–24:00 UTC
July
Jul 1 00:00–04:00, 08:00–12:00 UTC Jul 2 00:00–04:00, 07:00–12:00, 17:00–21:00 UTC Jul 3 06:00–12:00, 16:00–18:00 UTC Jul 4 03:00–04:00, 06:00–13:00, 17:00–20:00 UTC Jul 5 07:00–13:00, 16:00–20:00 UTC Jul 6 03:00–11:00 UTC Jul 8 00:00–05:00, 07:00–12:00 UTC Jul 9 00:00–05:00, 07:00–12:00 UTC Jul 10 01:00–12:00 UTC Jul 11 00:00–04:00, 07:00–15:00 UTC Jul 12 00:00–04:00, 08:00–15:00 UTC Jul 13 04:00–13:00 UTC Jul 14 01:00–05:00, 07:00–13:00 UTC Jul 15 00:00–03:00, 12:00–13:00, 19:00–20:00 UTC Jul 16 01:00–09:00, 11:00–13:00 UTC Jul 17 00:00–01:00, 04:00–05:00, 07:00–13:00 UTC Jul 18 01:00–07:00, 10:00–13:00 UTC Jul 19 00:00–15:00 UTC Jul 20 01:00–05:00, 07:00–10:00, 13:00–14:00, 21:00–22:00 UTC Jul 21 03:00–05:00, 11:00–21:00 UTC Jul 22 11:00–14:00, 20:00–22:00 UTC Jul 23 01:00–07:00, 12:00–14:00, 19:00–23:00 UTC Jul 24 01:00–02:00, 04:00–10:00 UTC Jul 25 11:00–13:00, 16:00–21:00 UTC Jul 26 01:00–05:00, 08:00–09:00, 12:00–13:00, 15:00–16:00 UTC Jul 27 01:00–02:00, 07:00–13:00, 19:00–20:00 UTC