Most often in March, 30 days.
Highest follower count seen that year: 64,654.
January
Jan 1 00:00–05:00, 18:00–24:00 UTC Jan 2 00:00–04:00 UTC Jan 3 18:00–24:00 UTC Jan 4 00:00–04:00, 19:00–24:00 UTC Jan 5 00:00–04:00, 19:00–24:00 UTC Jan 6 00:00–04:00, 19:00–24:00 UTC Jan 7 00:00–04:00, 19:00–24:00 UTC Jan 8 00:00–04:00, 19:00–23:00 UTC Jan 9 00:00–04:00, 19:00–24:00 UTC Jan 13 17:00–24:00 UTC Jan 14 00:00–04:00, 19:00–24:00 UTC Jan 15 00:00–04:00, 19:00–24:00 UTC Jan 16 00:00–04:00, 19:00–24:00 UTC Jan 17 00:00–05:00 UTC Jan 19 19:00–23:00 UTC Jan 20 19:00–24:00 UTC Jan 21 00:00–04:00, 19:00–24:00 UTC Jan 22 00:00–04:00, 19:00–24:00 UTC Jan 23 00:00–04:00, 19:00–24:00 UTC Jan 24 01:00–04:00, 19:00–23:00 UTC Jan 25 00:00–03:00 UTC Jan 26 19:00–24:00 UTC Jan 27 00:00–01:00, 17:00–24:00 UTC Jan 28 00:00–05:00 UTC Jan 30 19:00–24:00 UTC Jan 31 00:00–04:00, 19:00–24:00 UTC
February
Feb 1 17:00–24:00 UTC Feb 2 00:00–04:00, 19:00–24:00 UTC Feb 3 00:00–04:00, 19:00–24:00 UTC Feb 4 01:00–06:00 UTC Feb 6 19:00–24:00 UTC Feb 7 00:00–04:00, 17:00–24:00 UTC Feb 8 00:00–04:00, 18:00–24:00 UTC Feb 9 00:00–03:00, 19:00–24:00 UTC Feb 10 00:00–02:00, 19:00–24:00 UTC Feb 11 00:00–07:00 UTC Feb 12 19:00–24:00 UTC Feb 13 00:00–03:00, 18:00–23:00 UTC Feb 14 00:00–04:00, 18:00–24:00 UTC Feb 15 00:00–04:00, 18:00–24:00 UTC Feb 16 00:00–04:00, 18:00–24:00 UTC Feb 17 00:00–04:00, 19:00–24:00 UTC Feb 18 00:00–02:00, 17:00–23:00 UTC Feb 19 00:00–03:00, 19:00–23:00 UTC Feb 20 00:00–04:00, 19:00–24:00 UTC Feb 21 00:00–02:00 UTC Feb 23 19:00–24:00 UTC Feb 24 00:00–05:00, 19:00–24:00 UTC Feb 25 00:00–05:00, 19:00–24:00 UTC Feb 26 00:00–04:00, 20:00–24:00 UTC Feb 27 00:00–05:00, 19:00–24:00 UTC Feb 28 00:00–04:00, 19:00–24:00 UTC
March
Mar 1 00:00–05:00, 19:00–23:00 UTC Mar 2 18:00–24:00 UTC Mar 3 00:00–04:00, 19:00–24:00 UTC Mar 4 00:00–05:00 UTC Mar 5 19:00–24:00 UTC Mar 6 00:00–04:00, 19:00–24:00 UTC Mar 7 01:00–04:00 UTC Mar 8 18:00–24:00 UTC Mar 9 00:00–07:00, 18:00–24:00 UTC Mar 10 00:00–05:00, 19:00–24:00 UTC Mar 11 00:00–04:00, 19:00–24:00 UTC Mar 12 00:00–04:00, 19:00–24:00 UTC Mar 13 01:00–02:00, 19:00–24:00 UTC Mar 14 00:00–04:00, 19:00–24:00 UTC Mar 15 00:00–05:00, 19:00–23:00 UTC Mar 16 19:00–24:00 UTC Mar 17 00:00–04:00, 18:00–23:00 UTC Mar 18 00:00–05:00 UTC Mar 19 19:00–24:00 UTC Mar 20 00:00–04:00, 19:00–24:00 UTC Mar 21 00:00–04:00 UTC Mar 22 19:00–24:00 UTC Mar 23 00:00–05:00, 19:00–24:00 UTC Mar 24 00:00–02:00 UTC Mar 26 19:00–24:00 UTC Mar 27 00:00–04:00, 19:00–24:00 UTC Mar 28 00:00–05:00, 19:00–24:00 UTC Mar 29 00:00–03:00, 18:00–24:00 UTC Mar 30 00:00–04:00, 18:00–24:00 UTC Mar 31 00:00–04:00, 19:00–24:00 UTC
April
Apr 1 00:00–03:00, 18:00–24:00 UTC Apr 2 00:00–03:00, 18:00–24:00 UTC Apr 3 00:00–04:00 UTC Apr 4 18:00–24:00 UTC Apr 5 00:00–06:00, 18:00–24:00 UTC Apr 6 00:00–04:00, 17:00–24:00 UTC Apr 7 00:00–03:00, 18:00–24:00 UTC Apr 8 00:00–03:00, 18:00–23:00 UTC Apr 9 00:00–03:00, 18:00–19:00, 22:00–24:00 UTC Apr 10 00:00–03:00, 18:00–21:00 UTC Apr 14 18:00–24:00 UTC Apr 15 00:00–03:00, 18:00–24:00 UTC Apr 16 00:00–03:00, 18:00–24:00 UTC Apr 17 00:00–04:00, 18:00–24:00 UTC Apr 18 00:00–04:00, 18:00–24:00 UTC Apr 19 00:00–04:00 UTC Apr 20 18:00–24:00 UTC Apr 21 00:00–03:00, 18:00–24:00 UTC Apr 22 00:00–04:00, 18:00–24:00 UTC Apr 23 00:00–03:00, 18:00–23:00 UTC Apr 24 00:00–04:00 UTC Apr 27 18:00–24:00 UTC Apr 28 00:00–03:00, 18:00–24:00 UTC Apr 29 00:00–03:00, 18:00–24:00 UTC Apr 30 00:00–02:00, 17:00–24:00 UTC
May
May 1 00:00–03:00, 18:00–24:00 UTC May 2 00:00–03:00 UTC May 4 18:00–24:00 UTC May 5 00:00–03:00, 18:00–24:00 UTC May 6 00:00–03:00, 18:00–24:00 UTC May 7 00:00–03:00, 18:00–22:00 UTC May 8 01:00–03:00, 18:00–24:00 UTC May 9 00:00–02:00, 18:00–24:00 UTC May 10 00:00–04:00 UTC May 11 18:00–24:00 UTC May 12 00:00–03:00, 18:00–24:00 UTC May 13 00:00–03:00, 14:00–24:00 UTC May 14 00:00–03:00, 18:00–24:00 UTC May 15 00:00–03:00, 18:00–24:00 UTC May 16 00:00–03:00 UTC May 18 17:00–24:00 UTC May 19 00:00–03:00, 17:00–24:00 UTC May 20 00:00–03:00, 18:00–24:00 UTC May 21 01:00–03:00 UTC May 23 17:00–24:00 UTC May 24 01:00–03:00, 17:00–24:00 UTC May 25 00:00–03:00, 18:00–24:00 UTC May 26 00:00–03:00, 18:00–24:00 UTC May 27 00:00–02:00, 18:00–24:00 UTC May 28 00:00–03:00, 18:00–24:00 UTC May 29 00:00–03:00, 14:00–24:00 UTC May 30 00:00–03:00, 18:00–24:00 UTC May 31 00:00–03:00, 18:00–24:00 UTC
June
Jun 1 00:00–03:00 UTC Jun 2 17:00–24:00 UTC Jun 3 00:00–03:00, 17:00–24:00 UTC Jun 4 00:00–03:00, 16:00–24:00 UTC Jun 5 01:00–04:00, 18:00–24:00 UTC Jun 6 00:00–03:00 UTC Jun 8 17:00–24:00 UTC Jun 9 00:00–03:00, 18:00–24:00 UTC Jun 10 00:00–03:00, 18:00–24:00 UTC Jun 11 00:00–03:00, 18:00–24:00 UTC Jun 12 00:00–03:00, 18:00–23:00 UTC Jun 13 18:00–19:00 UTC Jun 15 01:00–02:00, 17:00–24:00 UTC Jun 16 00:00–03:00, 17:00–24:00 UTC Jun 17 00:00–03:00, 18:00–23:00 UTC Jun 20 18:00–24:00 UTC Jun 21 00:00–05:00, 18:00–24:00 UTC Jun 22 00:00–05:00, 17:00–24:00 UTC Jun 23 00:00–01:00, 18:00–24:00 UTC Jun 24 00:00–03:00, 17:00–24:00 UTC Jun 25 00:00–01:00 UTC Jun 26 18:00–24:00 UTC Jun 27 00:00–03:00, 18:00–24:00 UTC Jun 28 00:00–03:00, 17:00–24:00 UTC Jun 29 00:00–02:00, 17:00–24:00 UTC Jun 30 00:00–03:00, 18:00–24:00 UTC
July
Jul 1 00:00–03:00, 18:00–24:00 UTC Jul 2 00:00–03:00, 18:00–24:00 UTC Jul 3 19:00–24:00 UTC Jul 4 00:00–02:00, 18:00–24:00 UTC Jul 5 00:00–03:00 UTC Jul 6 18:00–24:00 UTC Jul 7 00:00–01:00 UTC Jul 23 19:00–24:00 UTC Jul 24 00:00–04:00 UTC Jul 25 18:00–24:00 UTC Jul 26 00:00–02:00, 18:00–24:00 UTC Jul 27 00:00–03:00, 19:00–21:00 UTC