Most often in February, 26 days.
Highest follower count seen that year: 54,826.
January
Jan 1 00:00–02:00 UTC Jan 2 22:00–23:00 UTC Jan 3 23:00–24:00 UTC Jan 4 00:00–02:00, 21:00–24:00 UTC Jan 5 00:00–03:00 UTC Jan 6 04:00–07:00, 17:00–21:00 UTC Jan 7 05:00–06:00, 19:00–20:00 UTC Jan 8 19:00–23:00 UTC Jan 9 03:00–05:00, 19:00–23:00 UTC Jan 13 16:00–18:00, 22:00–23:00 UTC Jan 14 03:00–06:00, 19:00–22:00 UTC Jan 15 18:00–22:00 UTC Jan 16 04:00–07:00 UTC Jan 18 18:00–21:00 UTC Jan 19 19:00–22:00 UTC Jan 20 02:00–05:00, 19:00–21:00 UTC Jan 21 18:00–19:00 UTC Jan 22 15:00–17:00, 19:00–23:00 UTC Jan 23 03:00–06:00, 19:00–23:00 UTC Jan 24 21:00–24:00 UTC Jan 25 03:00–05:00, 18:00–21:00 UTC Jan 26 02:00–03:00, 06:00–07:00 UTC Jan 28 18:00–21:00 UTC Jan 29 04:00–06:00, 19:00–23:00 UTC Jan 30 19:00–22:00 UTC Jan 31 03:00–06:00, 16:00–23:00 UTC
February
Feb 1 17:00–23:00 UTC Feb 2 15:00–20:00 UTC Feb 3 16:00–22:00 UTC Feb 4 03:00–05:00 UTC Feb 5 19:00–21:00 UTC Feb 6 21:00–24:00 UTC Feb 7 14:00–19:00 UTC Feb 8 16:00–18:00 UTC Feb 9 21:00–23:00 UTC Feb 10 02:00–05:00, 18:00–21:00 UTC Feb 11 03:00–06:00, 15:00–18:00 UTC Feb 12 02:00–04:00 UTC Feb 13 20:00–21:00 UTC Feb 14 02:00–07:00, 23:00–24:00 UTC Feb 15 00:00–03:00 UTC Feb 16 17:00–23:00 UTC Feb 17 18:00–23:00 UTC Feb 18 03:00–05:00, 16:00–19:00 UTC Feb 19 03:00–05:00, 14:00–19:00 UTC Feb 21 14:00–18:00 UTC Feb 22 14:00–17:00, 22:00–24:00 UTC Feb 23 00:00–02:00, 18:00–22:00 UTC Feb 24 03:00–05:00, 20:00–22:00 UTC Feb 25 15:00–21:00 UTC Feb 26 01:00–05:00, 20:00–23:00 UTC Feb 27 06:00–07:00 UTC
March
Mar 1 00:00–03:00 UTC Mar 2 02:00–05:00, 14:00–20:00 UTC Mar 3 13:00–17:00 UTC Mar 4 03:00–05:00, 13:00–18:00 UTC Mar 5 13:00–18:00, 20:00–22:00 UTC Mar 6 19:00–21:00 UTC Mar 8 15:00–17:00, 23:00–24:00 UTC Mar 9 00:00–01:00 UTC Mar 10 15:00–19:00, 22:00–23:00 UTC Mar 11 04:00–06:00, 20:00–23:00 UTC Mar 12 03:00–04:00, 13:00–17:00, 22:00–23:00 UTC Mar 13 16:00–18:00 UTC Mar 14 05:00–10:00, 22:00–24:00 UTC Mar 15 00:00–01:00, 17:00–19:00 UTC Mar 16 19:00–23:00 UTC Mar 17 18:00–20:00 UTC Mar 18 02:00–06:00, 18:00–21:00 UTC Mar 19 17:00–21:00 UTC Mar 21 03:00–06:00, 20:00–21:00 UTC Mar 22 18:00–20:00 UTC Mar 24 21:00–23:00 UTC Mar 25 04:00–06:00 UTC Mar 27 18:00–19:00, 22:00–24:00 UTC Mar 28 06:00–08:00, 10:00–11:00 UTC Mar 30 18:00–22:00 UTC Mar 31 02:00–04:00, 14:00–16:00, 20:00–21:00 UTC
April
Apr 1 07:00–08:00 UTC Apr 2 14:00–15:00, 23:00–24:00 UTC Apr 3 00:00–01:00 UTC Apr 5 21:00–24:00 UTC Apr 6 17:00–20:00 UTC Apr 7 02:00–04:00 UTC Apr 8 18:00–21:00 UTC Apr 9 22:00–23:00 UTC Apr 11 18:00–21:00 UTC Apr 12 01:00–04:00, 18:00–23:00 UTC Apr 13 02:00–06:00, 16:00–20:00 UTC Apr 14 01:00–04:00, 14:00–18:00 UTC Apr 17 02:00–06:00, 18:00–20:00 UTC Apr 18 02:00–04:00, 20:00–23:00 UTC Apr 20 15:00–19:00 UTC Apr 21 20:00–23:00 UTC Apr 24 21:00–24:00 UTC Apr 25 03:00–05:00, 18:00–21:00 UTC Apr 27 19:00–20:00 UTC Apr 28 18:00–19:00 UTC Apr 29 03:00–04:00 UTC Apr 30 18:00–20:00 UTC
May
May 5 18:00–21:00 UTC May 6 03:00–07:00, 19:00–21:00 UTC May 7 17:00–22:00 UTC May 8 05:00–06:00 UTC May 11 16:00–17:00, 21:00–23:00 UTC May 12 03:00–04:00, 19:00–22:00 UTC May 13 05:00–07:00 UTC May 14 03:00–05:00 UTC May 15 04:00–06:00 UTC May 16 05:00–06:00 UTC May 21 06:00–08:00 UTC May 26 06:00–07:00, 11:00–12:00 UTC May 28 04:00–06:00 UTC May 29 04:00–06:00 UTC May 30 06:00–08:00 UTC May 31 04:00–06:00, 08:00–09:00 UTC
June
Jun 2 05:00–07:00 UTC Jun 3 07:00–09:00 UTC Jun 4 05:00–08:00 UTC Jun 5 05:00–08:00 UTC Jun 6 01:00–05:00 UTC Jun 7 06:00–08:00 UTC Jun 10 07:00–09:00 UTC Jun 11 06:00–09:00 UTC Jun 12 07:00–09:00 UTC Jun 13 08:00–10:00 UTC Jun 15 04:00–08:00 UTC Jun 16 04:00–08:00 UTC Jun 17 04:00–09:00 UTC Jun 18 08:00–10:00 UTC Jun 22 04:00–05:00 UTC Jun 23 04:00–08:00 UTC Jun 24 17:00–19:00, 21:00–22:00 UTC Jun 25 08:00–09:00 UTC Jun 26 18:00–22:00 UTC Jun 27 06:00–08:00 UTC Jun 28 16:00–18:00 UTC
July
Jul 1 07:00–08:00 UTC Jul 2 01:00–02:00 UTC Jul 3 03:00–05:00 UTC Jul 4 07:00–08:00, 20:00–21:00 UTC Jul 6 06:00–07:00 UTC Jul 7 05:00–07:00 UTC Jul 8 04:00–05:00 UTC Jul 10 04:00–06:00 UTC Jul 15 18:00–22:00 UTC Jul 16 03:00–05:00, 15:00–18:00, 20:00–22:00 UTC Jul 17 03:00–04:00, 20:00–23:00 UTC Jul 19 19:00–22:00 UTC Jul 20 20:00–22:00 UTC Jul 21 19:00–22:00 UTC Jul 22 15:00–20:00 UTC Jul 23 04:00–06:00, 19:00–23:00 UTC Jul 24 10:00–12:00 UTC Jul 27 19:00–21:00 UTC