Most often in March, 27 days.
Highest follower count seen that year: 14,382.
January
Jan 1 17:00–20:00 UTC Jan 2 10:00–11:00 UTC Jan 4 08:00–09:00 UTC Jan 5 10:00–12:00 UTC Jan 6 11:00–13:00, 19:00–20:00 UTC Jan 7 14:00–21:00 UTC Jan 8 11:00–12:00, 20:00–21:00, 23:00–24:00 UTC Jan 9 05:00–07:00, 16:00–18:00 UTC Jan 10 08:00–09:00 UTC Jan 11 02:00–03:00, 18:00–20:00 UTC Jan 13 01:00–03:00, 19:00–21:00 UTC Jan 14 03:00–04:00, 21:00–22:00 UTC Jan 16 04:00–06:00 UTC Jan 17 05:00–06:00 UTC Jan 19 04:00–06:00 UTC Jan 20 06:00–07:00 UTC Jan 21 04:00–05:00, 09:00–10:00 UTC Jan 22 04:00–05:00, 09:00–10:00 UTC Jan 23 08:00–10:00 UTC Jan 24 08:00–09:00, 15:00–17:00 UTC Jan 26 09:00–11:00, 21:00–22:00 UTC Jan 27 09:00–11:00, 18:00–19:00 UTC Jan 28 18:00–19:00, 23:00–24:00 UTC Jan 29 09:00–12:00, 19:00–20:00, 22:00–23:00 UTC
February
Feb 1 09:00–11:00, 18:00–20:00 UTC Feb 2 13:00–16:00 UTC Feb 3 22:00–24:00 UTC Feb 4 10:00–12:00 UTC Feb 5 16:00–18:00, 20:00–23:00 UTC Feb 7 02:00–03:00, 21:00–22:00 UTC Feb 9 11:00–12:00, 17:00–24:00 UTC Feb 10 00:00–01:00, 18:00–20:00, 22:00–23:00 UTC Feb 13 06:00–07:00, 20:00–24:00 UTC Feb 14 05:00–06:00 UTC Feb 16 00:00–04:00, 16:00–19:00 UTC Feb 18 01:00–02:00, 21:00–22:00 UTC Feb 19 14:00–16:00, 18:00–22:00 UTC Feb 20 05:00–06:00, 22:00–24:00 UTC Feb 21 21:00–23:00 UTC Feb 23 00:00–02:00 UTC Feb 24 04:00–06:00, 10:00–12:00 UTC Feb 25 04:00–09:00, 18:00–19:00 UTC Feb 26 07:00–08:00, 12:00–14:00 UTC Feb 27 20:00–22:00 UTC Feb 28 01:00–05:00, 09:00–10:00 UTC
March
Mar 1 18:00–19:00 UTC Mar 2 07:00–10:00 UTC Mar 3 11:00–13:00, 21:00–22:00 UTC Mar 7 23:00–24:00 UTC Mar 8 00:00–01:00, 09:00–11:00, 18:00–20:00 UTC Mar 9 00:00–01:00, 09:00–11:00, 18:00–20:00 UTC Mar 10 07:00–11:00, 21:00–23:00 UTC Mar 11 09:00–10:00 UTC Mar 12 00:00–01:00, 09:00–12:00, 18:00–20:00 UTC Mar 13 18:00–21:00 UTC Mar 14 01:00–02:00, 08:00–10:00 UTC Mar 15 17:00–19:00 UTC Mar 16 11:00–12:00, 20:00–22:00 UTC Mar 17 01:00–02:00, 09:00–11:00, 18:00–20:00 UTC Mar 18 01:00–02:00, 18:00–19:00, 23:00–24:00 UTC Mar 19 05:00–07:00, 20:00–23:00 UTC Mar 20 20:00–21:00 UTC Mar 21 00:00–01:00, 12:00–14:00 UTC Mar 22 22:00–24:00 UTC Mar 23 10:00–11:00, 23:00–24:00 UTC Mar 24 05:00–07:00, 19:00–20:00, 22:00–24:00 UTC Mar 25 21:00–23:00 UTC Mar 26 03:00–05:00 UTC Mar 28 05:00–06:00 UTC Mar 29 02:00–04:00 UTC Mar 30 01:00–03:00, 10:00–11:00, 17:00–18:00 UTC Mar 31 01:00–02:00, 07:00–08:00 UTC
April
Apr 1 23:00–24:00 UTC Apr 2 09:00–11:00, 13:00–14:00 UTC Apr 3 05:00–06:00, 10:00–12:00 UTC Apr 5 01:00–02:00, 10:00–11:00 UTC Apr 8 11:00–12:00 UTC Apr 9 10:00–11:00 UTC Apr 10 07:00–10:00 UTC Apr 12 16:00–17:00, 23:00–24:00 UTC Apr 13 00:00–01:00, 07:00–09:00 UTC Apr 14 06:00–09:00, 23:00–24:00 UTC Apr 15 04:00–06:00, 11:00–12:00 UTC Apr 16 13:00–14:00, 18:00–19:00, 22:00–23:00 UTC Apr 17 11:00–12:00, 17:00–19:00 UTC Apr 18 10:00–11:00, 22:00–23:00 UTC Apr 19 18:00–20:00, 22:00–23:00 UTC Apr 20 06:00–07:00, 19:00–20:00 UTC Apr 22 07:00–10:00 UTC Apr 24 18:00–19:00 UTC Apr 26 17:00–18:00 UTC Apr 27 11:00–13:00, 20:00–22:00 UTC Apr 28 04:00–05:00, 20:00–22:00 UTC Apr 30 12:00–15:00, 20:00–23:00 UTC
May
May 1 01:00–03:00, 22:00–23:00 UTC May 3 07:00–08:00 UTC May 5 03:00–04:00 UTC May 6 23:00–24:00 UTC May 7 05:00–06:00 UTC May 8 05:00–08:00 UTC May 10 03:00–06:00 UTC May 11 09:00–12:00 UTC May 12 06:00–08:00, 18:00–20:00 UTC May 13 05:00–07:00, 17:00–18:00 UTC May 14 01:00–02:00, 19:00–20:00 UTC May 15 09:00–13:00 UTC May 16 05:00–08:00, 18:00–20:00 UTC May 18 07:00–09:00, 18:00–20:00, 22:00–23:00 UTC May 19 04:00–06:00, 20:00–21:00 UTC May 20 05:00–08:00, 10:00–11:00, 18:00–19:00 UTC May 21 08:00–10:00, 19:00–20:00 UTC May 22 17:00–18:00, 23:00–24:00 UTC May 24 00:00–01:00, 09:00–11:00, 15:00–17:00, 21:00–23:00 UTC May 25 10:00–11:00, 15:00–17:00, 20:00–21:00 UTC May 26 20:00–22:00 UTC May 27 12:00–13:00 UTC May 28 11:00–12:00, 17:00–18:00 UTC May 29 10:00–14:00, 21:00–22:00 UTC May 30 17:00–18:00, 21:00–23:00 UTC May 31 04:00–05:00, 18:00–20:00 UTC
June
Jun 1 09:00–11:00, 16:00–17:00, 19:00–20:00 UTC Jun 2 00:00–01:00, 20:00–22:00 UTC Jun 3 05:00–06:00, 18:00–21:00 UTC Jun 4 07:00–08:00, 18:00–19:00, 23:00–24:00 UTC Jun 5 00:00–01:00, 23:00–24:00 UTC Jun 6 00:00–02:00 UTC Jun 7 01:00–03:00 UTC Jun 8 00:00–01:00, 18:00–19:00, 23:00–24:00 UTC Jun 10 18:00–19:00 UTC Jun 11 02:00–03:00 UTC Jun 12 04:00–07:00 UTC Jun 13 03:00–04:00 UTC Jun 15 04:00–06:00 UTC Jun 16 02:00–03:00 UTC Jun 17 04:00–05:00, 09:00–11:00 UTC Jun 20 09:00–11:00, 14:00–15:00 UTC Jun 21 06:00–07:00, 09:00–10:00 UTC Jun 22 04:00–06:00, 09:00–10:00 UTC Jun 23 06:00–07:00, 12:00–13:00, 23:00–24:00 UTC Jun 24 00:00–01:00, 09:00–10:00, 12:00–13:00 UTC Jun 25 00:00–02:00, 20:00–22:00 UTC Jun 26 06:00–07:00, 09:00–11:00, 19:00–20:00 UTC Jun 28 05:00–10:00 UTC Jun 29 04:00–06:00, 12:00–14:00 UTC Jun 30 05:00–07:00, 23:00–24:00 UTC
July
Jul 1 07:00–08:00, 11:00–12:00, 14:00–15:00 UTC Jul 3 08:00–10:00 UTC Jul 4 02:00–03:00, 08:00–09:00, 13:00–14:00 UTC Jul 5 08:00–10:00 UTC Jul 6 06:00–07:00, 09:00–11:00, 14:00–15:00 UTC Jul 7 01:00–02:00, 10:00–11:00 UTC Jul 8 00:00–01:00, 08:00–09:00, 11:00–12:00, 15:00–16:00 UTC Jul 9 00:00–01:00, 06:00–08:00, 10:00–11:00 UTC Jul 10 04:00–05:00, 17:00–18:00 UTC Jul 11 01:00–03:00, 07:00–08:00, 17:00–18:00 UTC Jul 12 01:00–02:00, 06:00–08:00 UTC Jul 13 11:00–14:00, 17:00–19:00 UTC Jul 15 00:00–01:00, 08:00–09:00, 18:00–19:00, 23:00–24:00 UTC Jul 16 06:00–07:00, 10:00–11:00, 15:00–16:00, 18:00–19:00, 22:00–24:00 UTC Jul 17 03:00–04:00, 06:00–07:00, 16:00–17:00 UTC Jul 18 02:00–05:00, 07:00–08:00, 12:00–13:00 UTC Jul 19 06:00–07:00 UTC Jul 21 05:00–07:00, 12:00–13:00, 15:00–17:00 UTC Jul 22 14:00–15:00, 17:00–19:00 UTC Jul 23 07:00–08:00, 23:00–24:00 UTC Jul 24 09:00–10:00 UTC Jul 25 11:00–15:00 UTC Jul 26 03:00–04:00, 12:00–14:00 UTC Jul 27 14:00–15:00 UTC