Most often in March, 30 days.
Highest follower count seen that year: 14,350.
January
Jan 2 04:00–08:00 UTC Jan 4 05:00–07:00 UTC Jan 5 04:00–06:00, 14:00–19:00 UTC Jan 6 02:00–06:00, 15:00–19:00 UTC Jan 7 03:00–06:00 UTC Jan 8 04:00–06:00, 14:00–20:00 UTC Jan 9 04:00–07:00, 14:00–19:00 UTC Jan 10 05:00–08:00, 18:00–20:00 UTC Jan 11 05:00–09:00, 16:00–17:00 UTC Jan 12 04:00–06:00, 14:00–19:00 UTC Jan 13 05:00–06:00, 14:00–19:00 UTC Jan 14 04:00–05:00, 14:00–18:00 UTC Jan 15 03:00–05:00, 14:00–16:00 UTC Jan 16 04:00–06:00, 14:00–19:00 UTC Jan 18 07:00–08:00, 18:00–19:00 UTC Jan 21 04:00–05:00, 14:00–19:00 UTC Jan 22 03:00–05:00, 14:00–19:00 UTC Jan 23 03:00–04:00, 14:00–17:00 UTC Jan 24 05:00–08:00 UTC Jan 25 05:00–08:00 UTC Jan 26 03:00–08:00 UTC Jan 27 03:00–05:00, 15:00–18:00 UTC Jan 28 15:00–18:00 UTC Jan 29 14:00–16:00, 18:00–19:00 UTC Jan 30 04:00–06:00 UTC Jan 31 05:00–09:00 UTC
February
Feb 2 04:00–06:00, 17:00–19:00 UTC Feb 3 04:00–06:00, 14:00–19:00 UTC Feb 4 04:00–05:00, 14:00–18:00, 21:00–22:00 UTC Feb 5 02:00–03:00, 11:00–12:00, 19:00–20:00 UTC Feb 6 04:00–05:00, 15:00–19:00 UTC Feb 7 06:00–07:00 UTC Feb 10 03:00–04:00, 14:00–17:00, 19:00–20:00 UTC Feb 11 03:00–04:00, 14:00–18:00 UTC Feb 12 03:00–05:00, 14:00–17:00 UTC Feb 13 14:00–19:00 UTC Feb 14 04:00–06:00 UTC Feb 15 04:00–07:00 UTC Feb 16 06:00–09:00, 16:00–17:00 UTC Feb 17 05:00–06:00 UTC Feb 18 16:00–22:00 UTC Feb 19 00:00–02:00, 05:00–07:00, 18:00–19:00 UTC Feb 20 05:00–07:00, 15:00–16:00, 20:00–21:00 UTC Feb 21 05:00–07:00, 16:00–17:00 UTC Feb 22 05:00–06:00 UTC Feb 23 06:00–08:00 UTC Feb 25 03:00–05:00, 14:00–19:00 UTC Feb 26 04:00–05:00, 15:00–17:00, 22:00–23:00 UTC Feb 27 02:00–04:00, 18:00–20:00, 23:00–24:00 UTC Feb 28 09:00–16:00, 19:00–20:00 UTC
March
Mar 1 06:00–08:00 UTC Mar 2 03:00–05:00, 14:00–17:00 UTC Mar 3 03:00–04:00, 15:00–18:00, 21:00–22:00 UTC Mar 4 04:00–05:00, 14:00–15:00 UTC Mar 5 05:00–07:00 UTC Mar 6 14:00–19:00 UTC Mar 7 14:00–15:00, 23:00–24:00 UTC Mar 8 00:00–01:00, 13:00–15:00, 20:00–21:00 UTC Mar 9 00:00–01:00, 13:00–18:00 UTC Mar 10 01:00–03:00, 13:00–17:00 UTC Mar 11 03:00–05:00, 14:00–18:00 UTC Mar 12 02:00–04:00, 13:00–18:00 UTC Mar 14 14:00–16:00, 20:00–21:00, 23:00–24:00 UTC Mar 15 00:00–02:00, 04:00–06:00, 15:00–18:00 UTC Mar 16 02:00–04:00, 13:00–18:00 UTC Mar 17 02:00–03:00, 13:00–19:00 UTC Mar 18 02:00–04:00, 10:00–11:00, 13:00–18:00, 20:00–22:00 UTC Mar 19 02:00–05:00, 13:00–18:00, 23:00–24:00 UTC Mar 20 03:00–08:00 UTC Mar 21 02:00–04:00, 19:00–23:00 UTC Mar 22 06:00–10:00, 13:00–17:00, 23:00–24:00 UTC Mar 23 00:00–03:00, 13:00–18:00 UTC Mar 24 04:00–07:00 UTC Mar 25 00:00–05:00, 13:00–18:00 UTC Mar 26 02:00–04:00, 13:00–18:00 UTC Mar 27 02:00–04:00, 13:00–16:00 UTC Mar 28 12:00–13:00, 15:00–16:00, 19:00–24:00 UTC Mar 29 00:00–01:00, 06:00–08:00, 14:00–15:00, 20:00–22:00 UTC Mar 30 02:00–04:00, 14:00–18:00 UTC Mar 31 16:00–18:00 UTC
April
Apr 1 02:00–03:00, 13:00–17:00 UTC Apr 2 06:00–08:00 UTC Apr 3 03:00–06:00 UTC Apr 4 03:00–05:00, 12:00–14:00 UTC Apr 5 03:00–05:00, 13:00–14:00, 20:00–21:00, 23:00–24:00 UTC Apr 6 01:00–02:00, 04:00–05:00, 13:00–15:00, 17:00–20:00 UTC Apr 10 02:00–06:00, 14:00–17:00, 20:00–21:00 UTC Apr 11 03:00–07:00, 13:00–15:00 UTC Apr 12 03:00–09:00, 14:00–16:00 UTC Apr 13 13:00–18:00, 21:00–24:00 UTC Apr 14 02:00–03:00, 13:00–19:00, 21:00–24:00 UTC Apr 15 00:00–04:00, 14:00–17:00, 19:00–23:00 UTC Apr 16 01:00–04:00, 10:00–11:00, 15:00–19:00 UTC Apr 17 02:00–05:00, 13:00–17:00, 19:00–21:00 UTC Apr 18 04:00–06:00, 14:00–23:00 UTC Apr 19 00:00–01:00, 05:00–09:00, 15:00–16:00, 22:00–23:00 UTC Apr 20 01:00–03:00, 14:00–19:00 UTC Apr 21 02:00–03:00, 10:00–11:00, 17:00–18:00, 21:00–23:00 UTC Apr 22 03:00–04:00, 09:00–11:00, 13:00–15:00, 17:00–21:00 UTC Apr 23 02:00–04:00, 13:00–18:00 UTC Apr 24 10:00–16:00 UTC Apr 25 01:00–04:00, 06:00–07:00, 16:00–19:00, 22:00–24:00 UTC Apr 26 03:00–09:00 UTC Apr 27 01:00–04:00, 14:00–18:00 UTC Apr 28 14:00–18:00 UTC Apr 29 14:00–18:00 UTC Apr 30 00:00–01:00, 13:00–17:00 UTC
May
May 1 02:00–06:00, 13:00–17:00 UTC May 3 06:00–07:00 UTC May 4 00:00–02:00 UTC May 6 00:00–03:00, 15:00–17:00 UTC May 7 01:00–03:00, 14:00–19:00 UTC May 8 13:00–18:00, 21:00–22:00 UTC May 9 03:00–04:00, 15:00–21:00 UTC May 10 04:00–05:00, 14:00–15:00, 19:00–21:00 UTC May 11 00:00–01:00, 13:00–14:00, 20:00–21:00 UTC May 12 00:00–02:00, 10:00–11:00, 14:00–18:00, 21:00–24:00 UTC May 13 01:00–02:00, 13:00–18:00 UTC May 14 13:00–16:00, 20:00–21:00 UTC May 15 13:00–18:00 UTC May 16 04:00–05:00, 14:00–15:00, 22:00–23:00 UTC May 17 02:00–06:00, 14:00–15:00, 21:00–23:00 UTC May 18 01:00–04:00, 13:00–17:00, 19:00–21:00 UTC May 19 00:00–03:00, 13:00–16:00, 21:00–22:00 UTC May 20 00:00–02:00, 11:00–12:00, 14:00–17:00, 20:00–21:00 UTC May 21 01:00–04:00, 16:00–18:00 UTC May 22 02:00–04:00, 13:00–19:00, 23:00–24:00 UTC May 23 03:00–09:00, 15:00–17:00 UTC May 24 14:00–19:00, 21:00–23:00 UTC May 25 01:00–06:00, 14:00–16:00, 23:00–24:00 UTC May 28 13:00–18:00, 22:00–24:00 UTC May 29 02:00–03:00, 13:00–18:00 UTC May 30 04:00–09:00, 15:00–22:00 UTC May 31 03:00–04:00, 14:00–17:00, 20:00–23:00 UTC
June
Jun 1 02:00–04:00, 13:00–15:00, 22:00–23:00 UTC Jun 2 01:00–04:00, 14:00–16:00 UTC Jun 3 14:00–15:00, 21:00–22:00 UTC Jun 4 03:00–05:00, 13:00–14:00, 17:00–18:00, 20:00–22:00 UTC Jun 5 01:00–08:00, 14:00–17:00, 22:00–24:00 UTC Jun 6 03:00–04:00, 06:00–08:00, 15:00–21:00, 23:00–24:00 UTC Jun 7 01:00–02:00, 04:00–08:00, 19:00–22:00 UTC Jun 8 02:00–04:00, 14:00–18:00, 21:00–24:00 UTC Jun 9 04:00–05:00, 09:00–10:00, 14:00–21:00 UTC Jun 10 03:00–05:00, 13:00–19:00, 22:00–23:00 UTC Jun 11 03:00–04:00, 13:00–19:00 UTC Jun 12 03:00–04:00, 14:00–15:00, 22:00–24:00 UTC Jun 13 00:00–04:00, 13:00–19:00 UTC Jun 15 02:00–05:00, 13:00–18:00, 22:00–23:00 UTC Jun 16 02:00–04:00, 13:00–16:00, 23:00–24:00 UTC Jun 17 02:00–03:00, 16:00–18:00, 20:00–21:00 UTC Jun 18 02:00–03:00, 12:00–18:00, 23:00–24:00 UTC Jun 19 13:00–21:00 UTC Jun 20 02:00–07:00, 18:00–19:00 UTC Jun 21 19:00–21:00, 23:00–24:00 UTC Jun 22 00:00–06:00, 14:00–18:00 UTC Jun 24 03:00–04:00 UTC Jun 25 13:00–21:00 UTC Jun 26 21:00–24:00 UTC Jun 27 00:00–01:00, 03:00–08:00, 13:00–19:00 UTC Jun 28 04:00–06:00 UTC Jun 30 16:00–17:00, 23:00–24:00 UTC
July
Jul 1 01:00–05:00, 11:00–14:00, 16:00–17:00, 19:00–24:00 UTC Jul 2 02:00–03:00 UTC Jul 5 00:00–01:00, 15:00–21:00, 23:00–24:00 UTC Jul 6 00:00–02:00, 04:00–05:00, 14:00–15:00, 19:00–22:00 UTC Jul 7 01:00–04:00, 21:00–23:00 UTC Jul 8 13:00–14:00 UTC Jul 11 18:00–24:00 UTC Jul 12 15:00–18:00, 22:00–24:00 UTC Jul 13 00:00–02:00, 14:00–20:00 UTC Jul 14 00:00–06:00, 13:00–16:00 UTC Jul 24 05:00–06:00 UTC