Most often in January, 26 days.
Highest follower count seen that year: 35,067.
January
Jan 1 01:00–05:00, 08:00–11:00 UTC Jan 3 20:00–22:00 UTC Jan 4 01:00–04:00, 20:00–24:00 UTC Jan 5 21:00–24:00 UTC Jan 6 16:00–19:00 UTC Jan 7 18:00–24:00 UTC Jan 9 21:00–24:00 UTC Jan 10 00:00–02:00, 04:00–06:00, 18:00–24:00 UTC Jan 11 18:00–24:00 UTC Jan 12 00:00–04:00, 10:00–11:00 UTC Jan 14 15:00–24:00 UTC Jan 15 12:00–13:00, 21:00–24:00 UTC Jan 16 00:00–01:00 UTC Jan 17 21:00–23:00 UTC Jan 18 21:00–23:00 UTC Jan 19 00:00–03:00, 06:00–08:00 UTC Jan 20 15:00–18:00 UTC Jan 21 12:00–14:00, 18:00–21:00 UTC Jan 22 18:00–19:00 UTC Jan 23 10:00–18:00 UTC Jan 24 17:00–20:00 UTC Jan 25 16:00–24:00 UTC Jan 26 00:00–01:00 UTC Jan 28 16:00–22:00 UTC Jan 29 00:00–02:00 UTC Jan 31 01:00–05:00, 14:00–16:00 UTC
February
Feb 2 16:00–20:00 UTC Feb 3 00:00–02:00, 10:00–12:00 UTC Feb 4 18:00–20:00 UTC Feb 6 01:00–09:00, 18:00–21:00 UTC Feb 8 02:00–08:00 UTC Feb 9 18:00–21:00 UTC Feb 10 18:00–21:00 UTC Feb 12 01:00–04:00, 06:00–07:00, 22:00–23:00 UTC Feb 13 17:00–21:00 UTC Feb 14 21:00–24:00 UTC Feb 15 01:00–05:00, 15:00–17:00, 19:00–22:00 UTC Feb 16 20:00–24:00 UTC Feb 17 01:00–03:00, 06:00–07:00, 20:00–21:00 UTC Feb 18 21:00–22:00 UTC Feb 19 00:00–03:00, 06:00–08:00, 12:00–14:00, 19:00–22:00 UTC Feb 20 10:00–12:00, 22:00–23:00 UTC Feb 21 20:00–24:00 UTC Feb 22 01:00–02:00, 04:00–08:00, 19:00–20:00 UTC Feb 23 21:00–24:00 UTC Feb 24 00:00–03:00 UTC Feb 27 21:00–24:00 UTC Feb 28 00:00–04:00, 21:00–23:00 UTC
March
Mar 1 13:00–21:00 UTC Mar 2 19:00–22:00 UTC Mar 4 01:00–02:00 UTC Mar 7 00:00–08:00 UTC Mar 8 17:00–19:00 UTC Mar 9 17:00–20:00 UTC Mar 11 22:00–24:00 UTC Mar 13 17:00–21:00 UTC Mar 14 02:00–04:00, 19:00–21:00 UTC Mar 15 00:00–02:00, 16:00–21:00 UTC Mar 16 01:00–02:00 UTC Mar 18 19:00–22:00 UTC Mar 19 01:00–02:00, 13:00–14:00, 19:00–21:00, 23:00–24:00 UTC Mar 20 00:00–01:00, 20:00–22:00 UTC Mar 21 00:00–01:00 UTC Mar 22 01:00–03:00, 21:00–24:00 UTC Mar 23 01:00–05:00, 18:00–24:00 UTC Mar 24 02:00–04:00, 11:00–12:00 UTC Mar 26 17:00–22:00 UTC Mar 27 17:00–22:00 UTC Mar 28 19:00–20:00 UTC
April
Apr 11 04:00–05:00, 16:00–17:00 UTC Apr 12 14:00–16:00 UTC Apr 13 18:00–21:00 UTC Apr 14 19:00–20:00 UTC Apr 15 20:00–23:00 UTC Apr 16 00:00–02:00, 18:00–20:00 UTC Apr 19 13:00–14:00, 19:00–21:00 UTC Apr 21 20:00–24:00 UTC Apr 22 02:00–03:00, 17:00–18:00, 22:00–24:00 UTC Apr 23 00:00–01:00, 12:00–13:00 UTC Apr 24 21:00–23:00 UTC Apr 25 01:00–06:00, 12:00–14:00 UTC Apr 26 16:00–18:00, 20:00–23:00 UTC Apr 27 00:00–03:00 UTC Apr 29 15:00–23:00 UTC Apr 30 16:00–18:00, 21:00–24:00 UTC
May
May 1 00:00–05:00 UTC May 2 03:00–16:00, 19:00–24:00 UTC May 3 00:00–02:00, 17:00–19:00, 23:00–24:00 UTC May 4 00:00–06:00 UTC May 8 15:00–21:00 UTC May 9 18:00–21:00 UTC May 10 00:00–06:00, 19:00–23:00 UTC May 14 15:00–18:00 UTC May 16 18:00–22:00 UTC May 17 18:00–19:00 UTC May 18 02:00–03:00 UTC May 23 17:00–19:00, 21:00–24:00 UTC May 24 01:00–05:00, 19:00–21:00 UTC May 25 21:00–24:00 UTC May 26 00:00–01:00, 03:00–04:00, 18:00–24:00 UTC May 27 16:00–21:00, 23:00–24:00 UTC May 28 00:00–01:00, 18:00–21:00, 23:00–24:00 UTC May 29 00:00–01:00, 18:00–24:00 UTC May 30 00:00–01:00, 18:00–21:00, 23:00–24:00 UTC May 31 00:00–01:00, 20:00–24:00 UTC
June
Jun 1 18:00–24:00 UTC Jun 2 17:00–20:00 UTC Jun 3 12:00–14:00, 17:00–24:00 UTC Jun 4 00:00–03:00, 22:00–24:00 UTC Jun 5 17:00–23:00 UTC Jun 6 21:00–22:00 UTC Jun 7 00:00–01:00, 03:00–05:00 UTC Jun 8 00:00–02:00, 22:00–24:00 UTC Jun 9 00:00–01:00 UTC Jun 12 10:00–15:00, 17:00–22:00 UTC Jun 13 02:00–03:00, 06:00–07:00, 11:00–12:00, 17:00–18:00 UTC Jun 15 18:00–21:00 UTC Jun 16 20:00–22:00 UTC Jun 18 21:00–24:00 UTC Jun 19 22:00–24:00 UTC Jun 20 02:00–07:00, 11:00–16:00, 18:00–20:00, 23:00–24:00 UTC Jun 21 00:00–01:00 UTC Jun 22 16:00–18:00, 20:00–24:00 UTC Jun 23 00:00–01:00, 19:00–21:00, 23:00–24:00 UTC Jun 24 00:00–04:00 UTC Jun 25 16:00–18:00 UTC Jun 26 18:00–19:00 UTC Jun 29 19:00–22:00 UTC Jun 30 22:00–24:00 UTC
July
Jul 1 17:00–22:00 UTC Jul 2 15:00–16:00, 18:00–21:00 UTC Jul 3 00:00–01:00, 19:00–24:00 UTC Jul 4 00:00–09:00 UTC Jul 5 18:00–23:00 UTC Jul 6 02:00–03:00, 09:00–24:00 UTC Jul 7 00:00–01:00, 18:00–21:00 UTC Jul 8 18:00–21:00 UTC Jul 9 19:00–24:00 UTC Jul 10 00:00–02:00, 17:00–24:00 UTC Jul 11 00:00–01:00, 17:00–24:00 UTC Jul 12 00:00–02:00, 07:00–11:00, 15:00–24:00 UTC Jul 13 00:00–01:00 UTC Jul 15 07:00–10:00, 17:00–19:00, 23:00–24:00 UTC Jul 16 00:00–02:00 UTC Jul 17 19:00–21:00, 23:00–24:00 UTC Jul 18 00:00–03:00 UTC Jul 19 20:00–24:00 UTC Jul 20 00:00–03:00, 09:00–10:00, 12:00–13:00, 18:00–20:00, 22:00–23:00 UTC Jul 21 20:00–24:00 UTC Jul 22 00:00–03:00 UTC Jul 23 07:00–09:00, 18:00–19:00, 21:00–22:00 UTC Jul 25 22:00–24:00 UTC Jul 26 00:00–03:00, 10:00–14:00, 15:00–17:00, 18:00–21:00, 23:00–24:00 UTC Jul 27 11:00–13:00, 16:00–17:00 UTC