Most often in February, 23 days.
Highest follower count seen that year: 56,342.
January
Jan 12 19:00–21:00 UTC Jan 13 03:00–05:00, 19:00–21:00 UTC Jan 14 01:00–02:00 UTC Jan 15 18:00–21:00 UTC Jan 16 01:00–03:00 UTC Jan 19 18:00–20:00 UTC Jan 20 00:00–02:00, 18:00–20:00 UTC Jan 21 01:00–04:00, 19:00–22:00 UTC Jan 22 01:00–03:00, 18:00–21:00, 23:00–24:00 UTC Jan 23 00:00–01:00, 19:00–21:00 UTC Jan 24 01:00–03:00 UTC Jan 26 19:00–22:00 UTC Jan 27 01:00–03:00, 18:00–21:00 UTC Jan 28 02:00–04:00, 17:00–20:00 UTC Jan 29 03:00–04:00, 18:00–21:00, 23:00–24:00 UTC Jan 30 00:00–01:00, 19:00–21:00 UTC
February
Feb 2 17:00–20:00 UTC Feb 3 02:00–04:00, 19:00–21:00 UTC Feb 4 02:00–05:00, 18:00–21:00 UTC Feb 5 02:00–04:00, 19:00–24:00 UTC Feb 6 00:00–01:00 UTC Feb 9 19:00–22:00 UTC Feb 10 02:00–04:00, 19:00–22:00 UTC Feb 11 03:00–06:00, 20:00–22:00 UTC Feb 12 04:00–07:00, 21:00–22:00 UTC Feb 13 03:00–06:00, 23:00–24:00 UTC Feb 14 00:00–03:00 UTC Feb 16 23:00–24:00 UTC Feb 17 00:00–03:00 UTC Feb 18 00:00–03:00 UTC Feb 19 02:00–06:00 UTC Feb 20 03:00–06:00 UTC Feb 21 01:00–04:00 UTC Feb 22 03:00–05:00 UTC Feb 24 00:00–05:00 UTC Feb 25 01:00–04:00 UTC Feb 26 04:00–06:00 UTC Feb 27 02:00–04:00, 06:00–07:00 UTC Feb 28 03:00–06:00 UTC
March
Mar 3 00:00–04:00, 23:00–24:00 UTC Mar 4 00:00–03:00 UTC Mar 5 02:00–04:00, 06:00–07:00 UTC Mar 6 03:00–06:00 UTC Mar 7 02:00–05:00 UTC Mar 9 19:00–21:00 UTC Mar 10 02:00–04:00, 18:00–21:00 UTC Mar 11 03:00–05:00, 20:00–22:00 UTC Mar 12 02:00–05:00, 20:00–23:00 UTC Mar 13 03:00–04:00, 18:00–20:00 UTC Mar 14 02:00–04:00 UTC Mar 16 18:00–20:00 UTC Mar 17 18:00–20:00 UTC Mar 18 00:00–03:00, 17:00–20:00, 22:00–24:00 UTC Mar 19 00:00–01:00, 18:00–20:00, 22:00–24:00 UTC Mar 31 00:00–02:00, 14:00–15:00, 18:00–20:00 UTC
April
Apr 1 00:00–02:00 UTC Apr 4 01:00–03:00, 16:00–18:00, 21:00–24:00 UTC Apr 5 17:00–19:00, 23:00–24:00 UTC Apr 6 00:00–01:00, 20:00–23:00 UTC Apr 7 17:00–20:00, 23:00–24:00 UTC Apr 8 00:00–01:00 UTC Apr 10 16:00–19:00, 21:00–23:00 UTC Apr 11 16:00–19:00, 21:00–24:00 UTC Apr 12 20:00–23:00 UTC Apr 13 17:00–24:00 UTC Apr 14 17:00–20:00, 22:00–24:00 UTC Apr 15 22:00–24:00 UTC Apr 17 18:00–20:00, 23:00–24:00 UTC Apr 18 00:00–01:00, 16:00–23:00 UTC Apr 19 17:00–20:00 UTC Apr 20 01:00–03:00, 18:00–20:00, 23:00–24:00 UTC Apr 21 16:00–20:00 UTC Apr 24 17:00–20:00, 22:00–24:00 UTC Apr 25 00:00–01:00, 17:00–20:00, 22:00–24:00 UTC Apr 26 00:00–01:00, 20:00–23:00 UTC Apr 27 01:00–02:00, 17:00–21:00 UTC Apr 28 16:00–17:00, 20:00–23:00 UTC
May
May 1 16:00–22:00 UTC May 2 16:00–19:00, 23:00–24:00 UTC May 3 00:00–01:00, 16:00–19:00, 22:00–24:00 UTC May 4 00:00–01:00, 17:00–19:00, 22:00–24:00 UTC May 5 16:00–19:00, 21:00–22:00 UTC May 8 17:00–19:00, 23:00–24:00 UTC May 9 00:00–01:00, 20:00–24:00 UTC May 10 18:00–19:00, 22:00–24:00 UTC May 11 00:00–01:00, 17:00–19:00, 21:00–24:00 UTC May 12 16:00–19:00, 23:00–24:00 UTC May 13 00:00–01:00 UTC May 15 21:00–23:00 UTC May 16 01:00–02:00, 16:00–22:00 UTC May 17 19:00–22:00 UTC May 18 17:00–19:00 UTC May 19 16:00–19:00 UTC May 20 16:00–19:00 UTC May 27 21:00–23:00 UTC May 28 16:00–19:00 UTC May 29 00:00–03:00, 23:00–24:00 UTC May 30 00:00–01:00, 18:00–20:00 UTC May 31 00:00–02:00, 20:00–22:00 UTC
June
Jun 1 17:00–19:00, 22:00–24:00 UTC Jun 2 01:00–03:00, 22:00–24:00 UTC Jun 3 00:00–03:00 UTC Jun 9 19:00–21:00 UTC Jun 15 01:00–02:00, 17:00–19:00 UTC Jun 16 00:00–02:00, 17:00–20:00 UTC Jun 17 01:00–03:00 UTC Jun 19 18:00–20:00 UTC Jun 20 00:00–02:00, 16:00–19:00, 23:00–24:00 UTC Jun 21 00:00–01:00, 18:00–22:00 UTC Jun 22 02:00–05:00, 16:00–19:00, 21:00–23:00 UTC Jun 23 17:00–20:00, 22:00–24:00 UTC Jun 27 00:00–02:00 UTC Jun 28 01:00–03:00 UTC Jun 29 17:00–19:00 UTC Jun 30 03:00–05:00, 16:00–18:00 UTC
July
Jul 1 02:00–04:00, 19:00–21:00 UTC Jul 2 03:00–05:00 UTC Jul 3 21:00–24:00 UTC Jul 4 03:00–05:00 UTC Jul 7 20:00–22:00 UTC Jul 8 03:00–05:00, 21:00–24:00 UTC Jul 9 04:00–06:00 UTC Jul 10 00:00–02:00, 19:00–22:00 UTC Jul 11 20:00–23:00 UTC Jul 22 22:00–24:00 UTC Jul 23 19:00–21:00 UTC Jul 27 19:00–21:00 UTC