Most often in April, 30 days.
Highest follower count seen that year: 17,009.
January
Jan 1 04:00–08:00 UTC Jan 2 04:00–05:00, 19:00–21:00 UTC Jan 3 00:00–05:00 UTC Jan 4 01:00–02:00, 04:00–06:00, 13:00–15:00, 19:00–20:00, 22:00–24:00 UTC Jan 5 17:00–21:00, 23:00–24:00 UTC Jan 7 09:00–11:00, 17:00–19:00 UTC Jan 8 04:00–06:00, 17:00–20:00 UTC Jan 9 04:00–06:00, 17:00–19:00 UTC Jan 10 04:00–07:00, 17:00–19:00 UTC Jan 11 05:00–06:00, 19:00–21:00 UTC Jan 12 04:00–06:00, 13:00–15:00 UTC Jan 13 08:00–10:00, 19:00–24:00 UTC Jan 14 08:00–10:00 UTC Jan 15 05:00–06:00 UTC Jan 16 05:00–07:00, 17:00–18:00, 20:00–21:00 UTC Jan 17 09:00–11:00, 15:00–17:00, 22:00–24:00 UTC Jan 19 05:00–06:00, 16:00–17:00 UTC Jan 20 10:00–12:00, 21:00–24:00 UTC Jan 21 09:00–10:00, 14:00–15:00 UTC Jan 22 02:00–03:00, 11:00–12:00, 19:00–20:00 UTC Jan 23 04:00–05:00, 15:00–16:00 UTC Jan 24 11:00–12:00, 15:00–16:00, 20:00–22:00 UTC Jan 26 04:00–05:00, 14:00–16:00 UTC Jan 27 10:00–11:00, 14:00–15:00, 20:00–21:00 UTC Jan 28 15:00–18:00 UTC Jan 29 02:00–03:00, 15:00–18:00, 20:00–21:00 UTC Jan 30 14:00–16:00 UTC Jan 31 15:00–16:00, 18:00–19:00 UTC
February
Feb 1 00:00–01:00, 23:00–24:00 UTC Feb 2 01:00–02:00, 15:00–16:00, 23:00–24:00 UTC Feb 4 01:00–02:00 UTC Feb 5 02:00–03:00 UTC Feb 6 05:00–06:00 UTC Feb 7 01:00–03:00 UTC Feb 8 00:00–01:00 UTC Feb 9 04:00–06:00, 22:00–23:00 UTC Feb 10 00:00–01:00 UTC Feb 11 01:00–03:00 UTC Feb 13 02:00–03:00, 21:00–24:00 UTC Feb 16 04:00–05:00, 21:00–23:00 UTC Feb 17 07:00–09:00 UTC Feb 18 00:00–03:00, 14:00–15:00 UTC Feb 19 02:00–06:00, 18:00–19:00 UTC Feb 20 05:00–07:00, 17:00–18:00 UTC Feb 21 01:00–03:00, 22:00–24:00 UTC Feb 22 02:00–05:00 UTC Feb 23 04:00–06:00 UTC Feb 24 01:00–02:00, 22:00–24:00 UTC Feb 25 02:00–03:00, 18:00–19:00 UTC Feb 26 04:00–06:00 UTC Feb 27 05:00–07:00 UTC Feb 28 00:00–02:00, 10:00–11:00, 21:00–23:00 UTC
March
Mar 1 03:00–04:00 UTC Mar 2 04:00–05:00 UTC Mar 3 00:00–02:00, 13:00–14:00 UTC Mar 4 01:00–02:00 UTC Mar 5 02:00–03:00 UTC Mar 7 16:00–17:00, 23:00–24:00 UTC Mar 8 02:00–03:00, 19:00–20:00 UTC Mar 9 18:00–19:00 UTC Mar 10 01:00–02:00 UTC Mar 11 01:00–03:00, 05:00–06:00 UTC Mar 12 02:00–07:00, 19:00–20:00 UTC Mar 13 04:00–06:00, 19:00–20:00 UTC Mar 14 16:00–18:00 UTC Mar 15 03:00–05:00 UTC Mar 16 04:00–06:00, 19:00–21:00 UTC Mar 17 00:00–02:00 UTC Mar 18 02:00–03:00 UTC Mar 19 01:00–03:00, 22:00–24:00 UTC Mar 20 17:00–18:00, 22:00–24:00 UTC Mar 22 00:00–01:00, 22:00–24:00 UTC Mar 24 02:00–03:00 UTC Mar 25 01:00–04:00 UTC Mar 26 01:00–03:00, 15:00–16:00, 21:00–22:00 UTC Mar 27 00:00–01:00, 16:00–18:00 UTC Mar 28 05:00–06:00, 08:00–10:00 UTC Mar 29 01:00–04:00, 23:00–24:00 UTC Mar 30 06:00–07:00, 23:00–24:00 UTC Mar 31 00:00–01:00 UTC
April
Apr 1 00:00–01:00 UTC Apr 2 03:00–05:00, 17:00–19:00 UTC Apr 3 04:00–07:00, 12:00–13:00 UTC Apr 4 22:00–24:00 UTC Apr 5 07:00–09:00, 16:00–17:00 UTC Apr 6 03:00–05:00, 12:00–14:00, 20:00–24:00 UTC Apr 7 00:00–01:00, 12:00–13:00, 23:00–24:00 UTC Apr 8 00:00–02:00, 09:00–10:00 UTC Apr 9 01:00–03:00 UTC Apr 10 03:00–04:00, 06:00–07:00 UTC Apr 11 07:00–09:00, 17:00–19:00 UTC Apr 12 01:00–02:00, 07:00–08:00 UTC Apr 13 03:00–05:00, 20:00–23:00 UTC Apr 14 08:00–09:00, 13:00–14:00 UTC Apr 15 00:00–02:00, 12:00–13:00 UTC Apr 16 01:00–02:00, 17:00–19:00 UTC Apr 17 12:00–13:00, 17:00–18:00, 21:00–23:00 UTC Apr 18 17:00–18:00 UTC Apr 19 18:00–19:00 UTC Apr 20 03:00–04:00, 19:00–22:00 UTC Apr 21 07:00–08:00 UTC Apr 22 00:00–01:00, 09:00–11:00 UTC Apr 23 01:00–02:00 UTC Apr 24 15:00–17:00, 22:00–23:00 UTC Apr 25 09:00–10:00, 20:00–22:00 UTC Apr 26 21:00–22:00 UTC Apr 27 23:00–24:00 UTC Apr 28 10:00–11:00 UTC Apr 29 01:00–02:00, 12:00–14:00 UTC Apr 30 00:00–02:00, 15:00–16:00, 22:00–23:00 UTC
May
May 1 16:00–18:00, 22:00–23:00 UTC May 3 00:00–01:00, 19:00–20:00 UTC May 4 22:00–24:00 UTC May 6 01:00–03:00 UTC May 7 01:00–02:00, 18:00–19:00 UTC May 9 19:00–21:00 UTC May 11 21:00–22:00 UTC May 12 11:00–14:00, 22:00–23:00 UTC May 15 01:00–02:00, 16:00–17:00 UTC May 16 15:00–17:00 UTC May 17 02:00–03:00 UTC May 18 03:00–05:00, 12:00–13:00 UTC May 19 13:00–15:00, 20:00–24:00 UTC May 20 20:00–21:00 UTC May 22 04:00–05:00, 14:00–16:00 UTC May 23 03:00–04:00 UTC May 24 01:00–03:00 UTC May 25 03:00–04:00, 15:00–16:00, 23:00–24:00 UTC May 27 02:00–03:00 UTC May 28 18:00–19:00 UTC May 29 01:00–02:00 UTC May 30 02:00–03:00, 23:00–24:00 UTC May 31 17:00–18:00 UTC
June
Jun 9 23:00–24:00 UTC Jun 10 00:00–01:00, 03:00–04:00 UTC Jun 11 03:00–04:00, 18:00–19:00, 22:00–23:00 UTC Jun 13 00:00–03:00 UTC Jun 15 03:00–04:00, 14:00–15:00 UTC Jun 16 01:00–02:00 UTC Jun 17 01:00–03:00, 15:00–17:00 UTC Jun 18 01:00–02:00 UTC Jun 20 03:00–04:00 UTC Jun 21 01:00–02:00, 05:00–06:00 UTC Jun 22 03:00–04:00 UTC Jun 23 05:00–06:00 UTC Jun 24 02:00–03:00 UTC Jun 26 00:00–01:00, 03:00–04:00 UTC Jun 27 02:00–03:00 UTC Jun 28 01:00–02:00 UTC Jun 30 01:00–04:00 UTC
July
Jul 16 02:00–04:00 UTC Jul 18 05:00–06:00 UTC Jul 21 01:00–03:00, 21:00–23:00 UTC