Most often in March, 29 days.
Highest follower count seen that year: 14,141.
January
Jan 6 20:00–24:00 UTC Jan 7 00:00–05:00, 20:00–24:00 UTC Jan 8 00:00–04:00, 20:00–24:00 UTC Jan 9 00:00–04:00, 19:00–24:00 UTC Jan 10 00:00–04:00, 19:00–24:00 UTC Jan 11 00:00–05:00, 20:00–24:00 UTC Jan 12 00:00–05:00, 20:00–24:00 UTC Jan 13 00:00–05:00, 20:00–24:00 UTC Jan 14 00:00–04:00, 20:00–24:00 UTC Jan 15 00:00–05:00, 20:00–24:00 UTC Jan 16 00:00–05:00, 19:00–24:00 UTC Jan 17 00:00–04:00, 20:00–24:00 UTC Jan 18 00:00–05:00, 20:00–23:00 UTC Jan 19 00:00–04:00 UTC Jan 21 20:00–24:00 UTC Jan 22 00:00–01:00, 20:00–24:00 UTC Jan 23 00:00–04:00, 19:00–24:00 UTC Jan 24 00:00–05:00, 20:00–24:00 UTC Jan 25 00:00–04:00, 19:00–20:00, 22:00–24:00 UTC Jan 26 00:00–04:00, 20:00–23:00 UTC Jan 27 00:00–05:00, 20:00–24:00 UTC Jan 28 00:00–04:00, 20:00–23:00 UTC Jan 29 00:00–04:00, 20:00–24:00 UTC Jan 30 00:00–05:00, 19:00–24:00 UTC Jan 31 00:00–04:00, 21:00–24:00 UTC
February
Feb 1 00:00–01:00, 03:00–06:00, 20:00–24:00 UTC Feb 2 00:00–03:00 UTC Feb 3 20:00–24:00 UTC Feb 4 01:00–04:00 UTC Feb 5 20:00–24:00 UTC Feb 6 00:00–06:00, 22:00–23:00 UTC Feb 7 19:00–24:00 UTC Feb 8 00:00–04:00, 21:00–24:00 UTC Feb 9 00:00–05:00, 21:00–24:00 UTC Feb 10 00:00–04:00, 19:00–24:00 UTC Feb 11 00:00–04:00, 19:00–24:00 UTC Feb 12 00:00–04:00, 20:00–24:00 UTC Feb 13 00:00–04:00, 19:00–24:00 UTC Feb 14 00:00–04:00, 20:00–24:00 UTC Feb 15 00:00–04:00, 20:00–24:00 UTC Feb 16 00:00–04:00 UTC Feb 18 20:00–24:00 UTC Feb 19 00:00–04:00, 20:00–24:00 UTC Feb 20 00:00–04:00 UTC Feb 21 20:00–24:00 UTC Feb 22 00:00–06:00, 21:00–24:00 UTC Feb 23 00:00–05:00, 20:00–24:00 UTC Feb 24 00:00–04:00, 20:00–24:00 UTC Feb 25 00:00–04:00, 21:00–24:00 UTC Feb 26 20:00–24:00 UTC Feb 27 00:00–04:00, 20:00–24:00 UTC Feb 28 00:00–04:00, 20:00–24:00 UTC
March
Mar 1 00:00–05:00, 20:00–24:00 UTC Mar 2 00:00–04:00 UTC Mar 4 19:00–24:00 UTC Mar 5 01:00–03:00, 20:00–22:00 UTC Mar 6 22:00–24:00 UTC Mar 7 00:00–04:00, 20:00–24:00 UTC Mar 8 00:00–05:00, 21:00–24:00 UTC Mar 9 00:00–05:00 UTC Mar 10 20:00–24:00 UTC Mar 11 00:00–02:00, 21:00–24:00 UTC Mar 12 00:00–03:00, 21:00–24:00 UTC Mar 13 00:00–04:00, 20:00–24:00 UTC Mar 14 00:00–04:00, 20:00–24:00 UTC Mar 15 00:00–04:00, 21:00–24:00 UTC Mar 16 00:00–05:00, 20:00–21:00 UTC Mar 18 21:00–24:00 UTC Mar 19 00:00–04:00, 20:00–24:00 UTC Mar 20 00:00–05:00, 20:00–24:00 UTC Mar 21 00:00–05:00, 21:00–24:00 UTC Mar 22 00:00–03:00 UTC Mar 23 21:00–24:00 UTC Mar 24 00:00–03:00 UTC Mar 25 21:00–24:00 UTC Mar 26 00:00–05:00, 20:00–24:00 UTC Mar 27 00:00–05:00, 19:00–24:00 UTC Mar 28 00:00–02:00, 22:00–24:00 UTC Mar 29 00:00–05:00, 21:00–24:00 UTC Mar 30 00:00–03:00, 20:00–24:00 UTC Mar 31 00:00–04:00, 20:00–24:00 UTC
April
Apr 1 00:00–02:00, 19:00–24:00 UTC Apr 2 00:00–04:00, 18:00–24:00 UTC Apr 3 00:00–02:00 UTC Apr 4 01:00–03:00, 19:00–24:00 UTC Apr 5 00:00–04:00, 20:00–24:00 UTC Apr 6 00:00–03:00, 19:00–24:00 UTC Apr 7 00:00–04:00, 19:00–24:00 UTC Apr 8 00:00–03:00, 20:00–24:00 UTC Apr 9 00:00–04:00, 22:00–24:00 UTC Apr 10 00:00–03:00, 19:00–22:00 UTC Apr 12 20:00–24:00 UTC Apr 13 00:00–03:00, 21:00–24:00 UTC Apr 14 00:00–04:00, 20:00–24:00 UTC Apr 15 00:00–03:00, 20:00–22:00 UTC Apr 18 22:00–24:00 UTC Apr 19 00:00–02:00, 20:00–23:00 UTC Apr 20 20:00–24:00 UTC Apr 21 00:00–01:00, 20:00–24:00 UTC Apr 22 00:00–01:00, 21:00–24:00 UTC Apr 23 00:00–03:00, 20:00–24:00 UTC Apr 24 00:00–03:00, 20:00–24:00 UTC Apr 25 00:00–04:00, 20:00–24:00 UTC Apr 26 00:00–03:00 UTC Apr 27 21:00–24:00 UTC Apr 28 00:00–03:00, 20:00–24:00 UTC Apr 29 00:00–03:00, 20:00–23:00 UTC Apr 30 00:00–02:00, 20:00–23:00 UTC
May
May 3 21:00–24:00 UTC May 4 00:00–03:00, 20:00–24:00 UTC May 5 00:00–04:00, 20:00–22:00 UTC May 8 02:00–03:00 UTC May 9 20:00–24:00 UTC May 10 00:00–04:00 UTC May 11 21:00–22:00 UTC May 14 23:00–24:00 UTC May 15 00:00–02:00, 21:00–24:00 UTC May 16 00:00–05:00, 21:00–24:00 UTC May 17 00:00–03:00, 20:00–24:00 UTC May 18 00:00–04:00, 20:00–24:00 UTC May 19 00:00–04:00, 21:00–24:00 UTC May 20 00:00–05:00, 20:00–22:00 UTC May 21 01:00–04:00, 19:00–24:00 UTC May 22 00:00–04:00, 19:00–24:00 UTC May 23 00:00–04:00, 21:00–24:00 UTC May 24 00:00–03:00, 20:00–24:00 UTC May 25 00:00–04:00, 20:00–24:00 UTC May 26 00:00–04:00, 21:00–24:00 UTC May 27 00:00–04:00, 20:00–24:00 UTC May 28 00:00–04:00, 20:00–24:00 UTC May 29 00:00–03:00, 20:00–24:00 UTC May 30 00:00–03:00, 20:00–24:00 UTC May 31 00:00–04:00 UTC
June
Jun 1 19:00–24:00 UTC Jun 2 00:00–04:00, 19:00–24:00 UTC Jun 3 00:00–04:00 UTC Jun 5 21:00–24:00 UTC Jun 6 01:00–04:00, 20:00–24:00 UTC Jun 7 00:00–04:00, 21:00–24:00 UTC Jun 8 00:00–06:00, 20:00–24:00 UTC Jun 9 00:00–04:00, 21:00–24:00 UTC Jun 10 00:00–05:00, 22:00–24:00 UTC Jun 11 00:00–04:00, 20:00–24:00 UTC Jun 12 00:00–04:00, 20:00–24:00 UTC Jun 13 00:00–03:00 UTC Jun 15 01:00–04:00, 20:00–24:00 UTC Jun 16 19:00–24:00 UTC Jun 17 00:00–04:00, 21:00–24:00 UTC Jun 18 00:00–05:00, 20:00–24:00 UTC Jun 19 00:00–05:00, 20:00–24:00 UTC Jun 20 00:00–05:00, 20:00–24:00 UTC Jun 21 00:00–05:00, 20:00–24:00 UTC Jun 22 00:00–05:00, 20:00–24:00 UTC Jun 23 00:00–01:00 UTC Jun 24 20:00–24:00 UTC Jun 25 00:00–04:00, 20:00–24:00 UTC Jun 26 00:00–03:00, 20:00–24:00 UTC Jun 27 00:00–04:00, 20:00–24:00 UTC Jun 28 00:00–05:00, 19:00–24:00 UTC Jun 29 00:00–01:00 UTC Jun 30 19:00–22:00 UTC
July
Jul 3 21:00–24:00 UTC Jul 4 00:00–02:00, 20:00–24:00 UTC Jul 5 00:00–04:00, 22:00–24:00 UTC Jul 6 00:00–04:00, 21:00–24:00 UTC Jul 7 00:00–04:00, 21:00–24:00 UTC Jul 8 00:00–04:00, 20:00–24:00 UTC Jul 9 00:00–04:00, 21:00–24:00 UTC Jul 10 00:00–04:00 UTC Jul 12 20:00–24:00 UTC Jul 13 00:00–03:00, 19:00–24:00 UTC Jul 14 00:00–03:00, 21:00–24:00 UTC Jul 15 00:00–03:00, 20:00–24:00 UTC Jul 16 00:00–03:00, 20:00–24:00 UTC Jul 17 00:00–04:00, 21:00–24:00 UTC Jul 18 00:00–06:00, 20:00–24:00 UTC Jul 19 00:00–04:00, 20:00–24:00 UTC Jul 20 00:00–02:00, 09:00–10:00 UTC