Most often in March, 27 days.
Highest follower count seen that year: 87,438.
January
Jan 4 16:00–24:00 UTC Jan 5 00:00–02:00 UTC Jan 6 16:00–24:00 UTC Jan 7 00:00–02:00, 16:00–24:00 UTC Jan 8 00:00–02:00 UTC Jan 9 16:00–24:00 UTC Jan 10 00:00–01:00, 16:00–24:00 UTC Jan 11 00:00–01:00, 16:00–24:00 UTC Jan 12 00:00–02:00, 16:00–24:00 UTC Jan 13 00:00–03:00 UTC Jan 28 17:00–24:00 UTC Jan 29 00:00–01:00 UTC Jan 30 16:00–24:00 UTC Jan 31 00:00–01:00, 16:00–24:00 UTC
February
Feb 1 00:00–02:00, 17:00–24:00 UTC Feb 2 00:00–02:00 UTC Feb 3 16:00–24:00 UTC Feb 4 00:00–02:00, 16:00–24:00 UTC Feb 5 00:00–01:00 UTC Feb 7 16:00–24:00 UTC Feb 8 00:00–01:00, 17:00–24:00 UTC Feb 9 00:00–02:00 UTC Feb 10 16:00–24:00 UTC Feb 11 00:00–01:00 UTC Feb 12 16:00–24:00 UTC Feb 13 00:00–01:00, 17:00–24:00 UTC Feb 14 00:00–01:00, 16:00–24:00 UTC Feb 15 00:00–01:00, 16:00–24:00 UTC Feb 16 00:00–01:00 UTC Feb 17 16:00–24:00 UTC Feb 18 00:00–02:00, 16:00–24:00 UTC Feb 19 00:00–01:00, 16:00–24:00 UTC Feb 20 00:00–01:00, 17:00–24:00 UTC Feb 21 00:00–02:00 UTC Feb 23 16:00–24:00 UTC Feb 24 00:00–01:00 UTC Feb 25 16:00–24:00 UTC Feb 26 00:00–01:00 UTC Feb 27 16:00–24:00 UTC Feb 28 00:00–01:00, 16:00–24:00 UTC
March
Mar 1 00:00–01:00, 16:00–24:00 UTC Mar 2 00:00–01:00 UTC Mar 4 16:00–24:00 UTC Mar 5 00:00–01:00, 16:00–24:00 UTC Mar 6 00:00–01:00, 17:00–24:00 UTC Mar 7 00:00–01:00, 16:00–24:00 UTC Mar 8 00:00–01:00, 16:00–24:00 UTC Mar 9 00:00–01:00 UTC Mar 10 16:00–24:00 UTC Mar 11 00:00–01:00 UTC Mar 15 16:00–24:00 UTC Mar 16 00:00–01:00, 17:00–24:00 UTC Mar 17 00:00–01:00, 19:00–24:00 UTC Mar 18 00:00–01:00 UTC Mar 19 16:00–24:00 UTC Mar 20 00:00–02:00, 16:00–24:00 UTC Mar 21 00:00–01:00, 16:00–24:00 UTC Mar 22 00:00–01:00, 16:00–24:00 UTC Mar 23 00:00–01:00, 16:00–24:00 UTC Mar 24 00:00–02:00 UTC Mar 25 18:00–24:00 UTC Mar 26 00:00–02:00, 16:00–24:00 UTC Mar 27 00:00–01:00, 16:00–24:00 UTC Mar 28 00:00–01:00, 16:00–24:00 UTC Mar 29 00:00–01:00, 16:00–24:00 UTC Mar 30 00:00–01:00 UTC Mar 31 16:00–24:00 UTC
April
Apr 1 00:00–01:00, 16:00–24:00 UTC Apr 2 00:00–01:00, 16:00–24:00 UTC Apr 3 00:00–01:00 UTC Apr 4 16:00–24:00 UTC Apr 5 00:00–01:00, 16:00–24:00 UTC Apr 6 00:00–01:00, 16:00–24:00 UTC Apr 7 00:00–02:00 UTC Apr 9 16:00–24:00 UTC Apr 10 00:00–01:00, 16:00–24:00 UTC Apr 11 17:00–24:00 UTC Apr 12 00:00–01:00 UTC Apr 14 16:00–24:00 UTC Apr 15 00:00–01:00, 17:00–24:00 UTC Apr 16 00:00–01:00, 16:00–24:00 UTC Apr 17 00:00–01:00 UTC Apr 19 16:00–24:00 UTC Apr 20 00:00–01:00, 16:00–24:00 UTC Apr 21 00:00–01:00, 16:00–24:00 UTC Apr 23 16:00–21:00 UTC Apr 25 16:00–24:00 UTC Apr 26 00:00–02:00, 16:00–23:00 UTC Apr 27 00:00–01:00 UTC Apr 29 03:00–12:00 UTC Apr 30 03:00–12:00 UTC
May
May 1 03:00–12:00 UTC May 2 03:00–12:00 UTC May 3 03:00–12:00 UTC May 5 04:00–13:00 UTC May 6 03:00–12:00 UTC May 8 03:00–12:00 UTC May 9 04:00–12:00 UTC May 10 04:00–13:00 UTC May 12 03:00–12:00 UTC May 13 03:00–12:00 UTC May 15 03:00–12:00 UTC May 16 03:00–11:00 UTC May 17 04:00–12:00 UTC May 18 03:00–12:00 UTC May 21 04:00–12:00 UTC May 23 03:00–12:00 UTC May 24 04:00–12:00 UTC May 27 03:00–12:00 UTC May 28 04:00–12:00 UTC May 29 03:00–12:00 UTC May 31 03:00–12:00 UTC
June
Jun 2 03:00–12:00 UTC Jun 4 08:00–16:00 UTC Jun 5 04:00–09:00 UTC Jun 6 05:00–14:00 UTC Jun 9 03:00–13:00 UTC Jun 10 03:00–12:00 UTC Jun 12 03:00–13:00 UTC Jun 13 04:00–13:00 UTC Jun 16 04:00–12:00 UTC Jun 17 04:00–12:00 UTC Jun 19 03:00–12:00 UTC Jun 20 03:00–12:00 UTC Jun 21 03:00–12:00 UTC Jun 23 04:00–12:00 UTC Jun 26 04:00–12:00 UTC Jun 27 03:00–13:00 UTC Jun 28 05:00–13:00 UTC
July
Jul 1 03:00–13:00 UTC Jul 2 03:00–12:00 UTC Jul 3 03:00–12:00 UTC Jul 4 03:00–12:00 UTC Jul 5 04:00–12:00 UTC Jul 7 03:00–12:00 UTC Jul 8 03:00–12:00 UTC Jul 10 03:00–12:00 UTC Jul 11 03:00–12:00 UTC Jul 12 03:00–12:00 UTC Jul 13 03:00–12:00 UTC Jul 15 04:00–13:00 UTC Jul 16 03:00–13:00 UTC Jul 17 04:00–11:00 UTC Jul 18 04:00–12:00 UTC Jul 21 04:00–13:00 UTC Jul 22 03:00–09:00 UTC Jul 24 05:00–12:00 UTC Jul 25 04:00–13:00 UTC Jul 26 04:00–13:00 UTC