Most often in January, 31 days.
Highest follower count seen that year: 46,258.
January
Jan 1 16:00–21:00 UTC Jan 2 16:00–21:00 UTC Jan 3 04:00–10:00, 19:00–20:00 UTC Jan 4 17:00–21:00 UTC Jan 5 04:00–08:00, 16:00–21:00 UTC Jan 6 16:00–21:00 UTC Jan 7 04:00–07:00, 16:00–22:00 UTC Jan 8 16:00–21:00 UTC Jan 9 17:00–20:00 UTC Jan 10 16:00–20:00 UTC Jan 11 04:00–07:00, 16:00–21:00 UTC Jan 12 16:00–21:00 UTC Jan 13 16:00–22:00 UTC Jan 14 16:00–21:00 UTC Jan 15 04:00–07:00, 16:00–20:00 UTC Jan 16 16:00–21:00 UTC Jan 17 04:00–07:00, 14:00–16:00 UTC Jan 18 14:00–20:00 UTC Jan 19 00:00–01:00, 16:00–21:00 UTC Jan 20 16:00–21:00 UTC Jan 21 16:00–20:00 UTC Jan 22 16:00–21:00 UTC Jan 23 03:00–07:00, 16:00–22:00 UTC Jan 24 17:00–22:00 UTC Jan 25 03:00–08:00, 17:00–21:00 UTC Jan 26 15:00–21:00 UTC Jan 27 03:00–07:00, 15:00–21:00 UTC Jan 28 16:00–24:00 UTC Jan 29 00:00–01:00, 17:00–21:00 UTC Jan 30 15:00–21:00 UTC Jan 31 02:00–06:00, 15:00–21:00 UTC
February
Feb 1 17:00–21:00 UTC Feb 2 02:00–06:00, 13:00–21:00 UTC Feb 3 17:00–23:00 UTC Feb 4 17:00–22:00 UTC Feb 5 16:00–23:00 UTC Feb 6 16:00–21:00 UTC Feb 8 15:00–22:00 UTC Feb 9 10:00–14:00, 16:00–21:00 UTC Feb 13 15:00–21:00 UTC Feb 14 14:00–21:00 UTC Feb 15 14:00–21:00 UTC Feb 17 08:00–12:00 UTC Feb 18 15:00–20:00 UTC Feb 20 11:00–12:00, 15:00–21:00 UTC Feb 21 16:00–20:00 UTC Feb 22 16:00–22:00 UTC Feb 23 18:00–22:00 UTC Feb 24 17:00–22:00 UTC Feb 25 16:00–22:00 UTC Feb 26 16:00–21:00 UTC Feb 27 16:00–22:00 UTC Feb 28 15:00–21:00 UTC
March
Mar 1 02:00–06:00, 16:00–21:00 UTC Mar 2 17:00–21:00 UTC Mar 3 16:00–20:00 UTC Mar 4 22:00–23:00 UTC Mar 5 19:00–20:00 UTC Mar 8 20:00–23:00 UTC Mar 9 20:00–23:00 UTC Mar 12 18:00–22:00 UTC Mar 13 17:00–23:00 UTC Mar 14 17:00–22:00 UTC Mar 15 02:00–06:00, 17:00–21:00 UTC Mar 16 17:00–22:00 UTC Mar 17 16:00–23:00 UTC Mar 18 17:00–23:00 UTC Mar 19 17:00–22:00 UTC Mar 20 16:00–21:00 UTC Mar 21 02:00–05:00, 19:00–21:00 UTC Mar 22 16:00–21:00 UTC Mar 23 02:00–06:00, 17:00–20:00 UTC Mar 24 17:00–21:00 UTC Mar 25 02:00–05:00, 19:00–21:00 UTC Mar 26 15:00–19:00, 23:00–24:00 UTC Mar 27 00:00–01:00, 17:00–22:00 UTC Mar 28 16:00–22:00 UTC Mar 29 02:00–05:00, 16:00–21:00 UTC Mar 30 17:00–20:00 UTC Mar 31 16:00–22:00 UTC
April
Apr 1 16:00–21:00 UTC Apr 2 16:00–23:00 UTC Apr 4 16:00–22:00 UTC Apr 5 17:00–22:00 UTC Apr 6 17:00–22:00 UTC Apr 7 16:00–22:00 UTC Apr 8 16:00–21:00 UTC Apr 9 02:00–05:00 UTC Apr 10 19:00–22:00 UTC Apr 11 13:00–20:00 UTC Apr 12 14:00–21:00 UTC Apr 13 14:00–21:00 UTC Apr 14 08:00–12:00 UTC Apr 15 09:00–14:00, 17:00–20:00 UTC Apr 16 08:00–12:00 UTC Apr 18 13:00–20:00 UTC Apr 19 06:00–11:00, 15:00–20:00 UTC Apr 20 07:00–10:00 UTC Apr 22 15:00–21:00 UTC Apr 23 15:00–21:00 UTC Apr 26 08:00–11:00, 15:00–21:00 UTC Apr 28 19:00–22:00 UTC Apr 29 17:00–20:00 UTC Apr 30 17:00–23:00 UTC
May
May 1 18:00–22:00 UTC May 2 18:00–21:00 UTC May 3 17:00–21:00 UTC May 4 02:00–04:00, 17:00–22:00 UTC May 5 17:00–22:00 UTC May 6 15:00–21:00 UTC May 7 02:00–05:00 UTC May 8 18:00–21:00 UTC May 9 16:00–22:00 UTC May 10 16:00–21:00 UTC May 11 02:00–04:00, 18:00–22:00 UTC May 12 17:00–21:00 UTC May 13 17:00–20:00 UTC May 14 18:00–23:00 UTC May 15 17:00–21:00 UTC May 16 17:00–22:00 UTC May 17 18:00–21:00 UTC May 18 02:00–05:00, 17:00–21:00 UTC May 19 16:00–21:00 UTC May 20 02:00–04:00, 17:00–22:00 UTC May 21 17:00–21:00 UTC May 22 02:00–05:00, 17:00–21:00 UTC May 23 17:00–22:00 UTC May 24 17:00–21:00 UTC May 25 16:00–21:00 UTC May 26 17:00–21:00 UTC May 28 17:00–22:00 UTC May 29 16:00–20:00 UTC May 30 17:00–22:00 UTC May 31 08:00–13:00, 16:00–21:00 UTC
June
Jun 11 18:00–22:00 UTC Jun 12 18:00–21:00 UTC Jun 13 17:00–19:00 UTC Jun 15 17:00–23:00 UTC Jun 16 17:00–22:00 UTC Jun 17 17:00–22:00 UTC Jun 18 17:00–21:00 UTC Jun 19 17:00–22:00 UTC Jun 20 17:00–22:00 UTC Jun 21 17:00–21:00 UTC Jun 22 17:00–24:00 UTC Jun 23 00:00–01:00, 18:00–22:00 UTC Jun 24 17:00–22:00 UTC Jun 25 17:00–22:00 UTC Jun 26 17:00–23:00 UTC Jun 27 17:00–21:00 UTC Jun 28 18:00–22:00 UTC Jun 29 17:00–23:00 UTC Jun 30 17:00–21:00 UTC
July
Jul 1 18:00–22:00 UTC Jul 2 17:00–21:00 UTC Jul 3 18:00–20:00 UTC Jul 4 17:00–22:00 UTC Jul 5 08:00–13:00, 17:00–21:00 UTC Jul 6 17:00–22:00 UTC Jul 8 16:00–21:00 UTC Jul 10 17:00–21:00 UTC Jul 11 16:00–21:00 UTC Jul 12 15:00–21:00 UTC Jul 14 18:00–22:00 UTC Jul 15 16:00–21:00 UTC Jul 16 16:00–22:00 UTC Jul 17 18:00–22:00 UTC Jul 18 18:00–21:00 UTC Jul 19 18:00–22:00 UTC Jul 20 18:00–22:00 UTC Jul 21 18:00–22:00 UTC Jul 22 18:00–22:00 UTC Jul 23 18:00–22:00 UTC Jul 24 17:00–22:00 UTC Jul 25 18:00–23:00 UTC Jul 26 17:00–21:00 UTC Jul 27 19:00–21:00 UTC