Most often in May, 21 days.
Highest follower count seen that year: 3,564.
January
Jan 14 14:00–24:00 UTC Jan 15 00:00–03:00, 15:00–24:00 UTC Jan 16 00:00–05:00 UTC Jan 17 02:00–05:00, 21:00–23:00 UTC Jan 18 02:00–06:00 UTC Jan 19 00:00–07:00 UTC Jan 20 02:00–07:00, 17:00–21:00 UTC Jan 23 00:00–05:00, 07:00–09:00 UTC Jan 24 02:00–03:00, 05:00–06:00, 23:00–24:00 UTC Jan 26 01:00–08:00 UTC Jan 27 00:00–07:00 UTC Jan 28 01:00–04:00, 23:00–24:00 UTC Jan 29 00:00–06:00 UTC Jan 30 04:00–05:00 UTC Jan 31 00:00–06:00, 19:00–24:00 UTC
February
Feb 1 00:00–01:00, 03:00–06:00 UTC Feb 3 05:00–06:00 UTC Feb 4 16:00–22:00 UTC Feb 5 01:00–03:00, 11:00–13:00 UTC Feb 6 01:00–05:00, 12:00–15:00 UTC Feb 7 00:00–06:00 UTC Feb 9 04:00–06:00 UTC Feb 10 02:00–06:00, 23:00–24:00 UTC Feb 11 00:00–06:00 UTC Feb 12 03:00–04:00 UTC Feb 13 00:00–02:00 UTC Feb 16 02:00–07:00, 21:00–23:00 UTC Feb 17 00:00–04:00, 13:00–16:00, 19:00–22:00 UTC Feb 18 00:00–04:00, 12:00–17:00, 21:00–22:00 UTC Feb 19 00:00–04:00 UTC Feb 20 01:00–04:00 UTC Feb 26 01:00–05:00, 22:00–23:00 UTC
March
Mar 29 02:00–05:00 UTC Mar 30 16:00–17:00, 22:00–24:00 UTC Mar 31 02:00–04:00, 11:00–16:00 UTC
April
Apr 1 02:00–04:00 UTC Apr 2 03:00–06:00 UTC Apr 3 02:00–03:00 UTC Apr 4 02:00–05:00 UTC Apr 5 20:00–24:00 UTC Apr 6 00:00–02:00 UTC Apr 8 00:00–02:00, 21:00–23:00 UTC Apr 9 02:00–04:00 UTC Apr 10 04:00–07:00 UTC Apr 12 01:00–06:00 UTC Apr 14 00:00–03:00 UTC Apr 15 02:00–05:00, 19:00–21:00, 23:00–24:00 UTC Apr 16 00:00–03:00 UTC Apr 17 02:00–05:00 UTC Apr 22 01:00–03:00 UTC Apr 23 00:00–04:00, 22:00–24:00 UTC Apr 24 00:00–04:00 UTC Apr 25 02:00–07:00 UTC Apr 26 00:00–05:00 UTC Apr 29 02:00–04:00 UTC
May
May 1 02:00–06:00 UTC May 5 23:00–24:00 UTC May 6 00:00–06:00 UTC May 7 01:00–05:00 UTC May 8 05:00–06:00, 23:00–24:00 UTC May 9 00:00–01:00, 03:00–06:00 UTC May 10 04:00–06:00 UTC May 12 03:00–07:00 UTC May 13 04:00–10:00 UTC May 15 05:00–07:00 UTC May 16 03:00–07:00, 14:00–20:00 UTC May 19 22:00–24:00 UTC May 20 00:00–05:00 UTC May 21 04:00–07:00 UTC May 22 02:00–08:00 UTC May 23 00:00–04:00 UTC May 27 02:00–04:00, 20:00–23:00 UTC May 28 01:00–06:00 UTC May 29 00:00–04:00, 10:00–15:00 UTC May 30 01:00–04:00, 18:00–19:00 UTC May 31 00:00–06:00 UTC
June
Jun 1 16:00–20:00, 23:00–24:00 UTC Jun 2 03:00–08:00, 22:00–23:00 UTC Jun 3 03:00–06:00 UTC Jun 4 01:00–07:00, 17:00–21:00 UTC Jun 5 03:00–08:00, 18:00–20:00 UTC Jun 6 02:00–07:00 UTC Jun 11 03:00–06:00 UTC Jun 12 02:00–06:00 UTC Jun 13 04:00–09:00 UTC
July
Jul 2 02:00–06:00, 17:00–20:00 UTC Jul 3 02:00–03:00 UTC Jul 4 03:00–05:00 UTC Jul 7 17:00–19:00 UTC Jul 8 01:00–02:00 UTC Jul 9 02:00–05:00 UTC Jul 10 04:00–05:00 UTC Jul 11 01:00–05:00, 15:00–18:00 UTC Jul 12 00:00–01:00 UTC Jul 15 23:00–24:00 UTC Jul 16 00:00–02:00, 16:00–17:00 UTC Jul 17 20:00–24:00 UTC Jul 18 00:00–03:00 UTC Jul 19 03:00–06:00 UTC Jul 21 16:00–24:00 UTC Jul 22 00:00–03:00, 17:00–22:00 UTC Jul 23 01:00–02:00 UTC Jul 24 18:00–20:00 UTC Jul 25 20:00–24:00 UTC Jul 26 00:00–03:00, 20:00–22:00, 23:00–24:00 UTC Jul 27 20:00–21:00 UTC