Most often in March, 29 days.
Highest follower count seen that year: 14,591.
January
Jan 4 03:00–05:00 UTC Jan 5 02:00–04:00 UTC Jan 6 03:00–05:00 UTC Jan 7 02:00–05:00, 14:00–20:00 UTC Jan 8 03:00–05:00 UTC Jan 9 03:00–04:00 UTC Jan 11 04:00–06:00 UTC Jan 12 03:00–05:00 UTC Jan 13 03:00–05:00 UTC Jan 14 03:00–04:00 UTC Jan 15 03:00–05:00 UTC Jan 16 03:00–05:00 UTC Jan 17 05:00–06:00 UTC Jan 18 04:00–05:00 UTC Jan 19 05:00–08:00 UTC Jan 20 03:00–05:00 UTC Jan 21 03:00–05:00 UTC Jan 23 03:00–05:00 UTC Jan 25 05:00–06:00 UTC Jan 26 03:00–06:00 UTC Jan 27 03:00–04:00 UTC Jan 28 02:00–05:00 UTC Jan 29 03:00–05:00 UTC Jan 30 03:00–04:00 UTC Jan 31 06:00–07:00 UTC
February
Feb 1 04:00–07:00, 19:00–20:00 UTC Feb 2 02:00–04:00 UTC Feb 3 03:00–04:00 UTC Feb 4 03:00–05:00 UTC Feb 5 03:00–05:00 UTC Feb 6 03:00–06:00, 16:00–19:00 UTC Feb 7 05:00–07:00 UTC Feb 9 04:00–05:00 UTC Feb 10 03:00–05:00 UTC Feb 11 02:00–05:00 UTC Feb 12 03:00–05:00 UTC Feb 13 03:00–05:00 UTC Feb 14 21:00–22:00 UTC Feb 15 03:00–06:00, 20:00–22:00 UTC Feb 20 04:00–06:00, 18:00–19:00 UTC Feb 21 05:00–07:00, 22:00–23:00 UTC Feb 22 03:00–06:00, 22:00–23:00 UTC Feb 23 03:00–05:00, 15:00–17:00 UTC Feb 24 03:00–06:00 UTC Feb 25 03:00–07:00, 18:00–19:00 UTC Feb 26 04:00–07:00 UTC
March
Mar 1 18:00–19:00 UTC Mar 3 02:00–04:00 UTC Mar 4 02:00–05:00 UTC Mar 5 03:00–05:00 UTC Mar 6 03:00–05:00 UTC Mar 7 06:00–08:00, 22:00–23:00 UTC Mar 8 18:00–20:00 UTC Mar 9 01:00–03:00 UTC Mar 10 02:00–04:00, 13:00–19:00 UTC Mar 11 01:00–03:00, 12:00–16:00, 18:00–19:00 UTC Mar 12 01:00–03:00 UTC Mar 13 02:00–03:00 UTC Mar 14 04:00–06:00, 12:00–13:00, 19:00–20:00 UTC Mar 16 01:00–03:00 UTC Mar 17 01:00–03:00 UTC Mar 18 01:00–03:00 UTC Mar 19 01:00–03:00 UTC Mar 20 02:00–04:00 UTC Mar 21 05:00–06:00 UTC Mar 22 17:00–19:00 UTC Mar 23 01:00–04:00 UTC Mar 24 02:00–04:00 UTC Mar 25 01:00–04:00 UTC Mar 26 02:00–04:00 UTC Mar 27 02:00–04:00 UTC Mar 28 04:00–06:00, 17:00–18:00 UTC Mar 29 16:00–17:00 UTC Mar 30 01:00–04:00 UTC Mar 31 02:00–03:00 UTC
April
Apr 1 02:00–04:00 UTC Apr 3 02:00–04:00 UTC Apr 5 20:00–22:00 UTC Apr 6 02:00–05:00, 13:00–15:00 UTC Apr 7 02:00–03:00 UTC Apr 8 02:00–04:00 UTC Apr 9 02:00–04:00 UTC Apr 10 02:00–04:00 UTC Apr 11 04:00–05:00 UTC Apr 12 05:00–07:00 UTC Apr 13 01:00–04:00 UTC Apr 14 02:00–04:00 UTC Apr 15 03:00–04:00 UTC Apr 16 02:00–04:00 UTC Apr 17 02:00–03:00 UTC Apr 18 01:00–03:00 UTC Apr 19 02:00–03:00, 20:00–21:00 UTC Apr 21 02:00–04:00 UTC Apr 22 02:00–05:00 UTC Apr 23 02:00–04:00, 15:00–17:00 UTC Apr 24 02:00–04:00 UTC Apr 25 02:00–03:00 UTC Apr 26 02:00–04:00, 16:00–17:00 UTC Apr 27 02:00–04:00 UTC Apr 28 01:00–03:00 UTC Apr 29 02:00–04:00 UTC Apr 30 01:00–04:00 UTC
May
May 1 02:00–04:00, 15:00–17:00 UTC May 2 03:00–04:00 UTC May 4 01:00–03:00 UTC May 5 02:00–03:00 UTC May 6 02:00–04:00 UTC May 7 01:00–04:00 UTC May 8 03:00–04:00 UTC May 9 02:00–05:00 UTC May 11 02:00–04:00, 12:00–15:00, 17:00–19:00 UTC May 12 01:00–04:00 UTC May 13 02:00–03:00 UTC May 14 02:00–04:00 UTC May 15 02:00–04:00 UTC May 16 04:00–05:00 UTC May 17 04:00–06:00 UTC May 19 02:00–04:00 UTC May 20 02:00–04:00 UTC May 21 02:00–04:00 UTC May 22 02:00–05:00 UTC May 23 05:00–06:00 UTC May 25 02:00–04:00 UTC May 26 01:00–03:00 UTC May 27 02:00–03:00 UTC May 28 02:00–04:00 UTC
June
Jun 2 02:00–03:00 UTC Jun 3 02:00–03:00 UTC Jun 4 02:00–04:00, 13:00–14:00 UTC Jun 5 02:00–04:00 UTC Jun 7 04:00–05:00 UTC Jun 8 02:00–04:00 UTC Jun 9 02:00–03:00 UTC Jun 10 02:00–04:00 UTC Jun 11 01:00–03:00 UTC Jun 13 05:00–06:00 UTC Jun 15 02:00–03:00 UTC Jun 16 03:00–04:00, 12:00–14:00 UTC Jun 17 02:00–03:00 UTC Jun 25 02:00–04:00 UTC
July
Jul 1 02:00–04:00 UTC Jul 2 02:00–04:00 UTC Jul 5 04:00–05:00, 17:00–18:00 UTC Jul 14 02:00–04:00 UTC Jul 15 03:00–04:00 UTC Jul 16 03:00–04:00, 12:00–13:00 UTC Jul 19 03:00–05:00 UTC Jul 21 03:00–05:00 UTC Jul 22 03:00–05:00 UTC Jul 24 05:00–07:00 UTC Jul 25 03:00–04:00 UTC Jul 26 02:00–04:00, 13:00–14:00 UTC