Most often in March, 29 days.
Highest follower count seen that year: 5,415.
January
Jan 3 03:00–10:00 UTC Jan 4 04:00–10:00, 20:00–24:00 UTC Jan 5 00:00–02:00 UTC Jan 6 11:00–18:00 UTC Jan 8 04:00–10:00 UTC Jan 9 04:00–10:00 UTC Jan 11 04:00–10:00 UTC Jan 15 04:00–10:00 UTC Jan 16 20:00–24:00 UTC Jan 17 00:00–03:00 UTC Jan 18 20:00–23:00 UTC Jan 19 00:00–02:00 UTC Jan 20 21:00–24:00 UTC Jan 21 00:00–03:00 UTC Jan 22 21:00–24:00 UTC Jan 23 00:00–03:00, 21:00–24:00 UTC Jan 24 00:00–03:00, 21:00–24:00 UTC Jan 25 01:00–03:00, 22:00–24:00 UTC Jan 26 00:00–03:00 UTC Jan 27 04:00–10:00 UTC Jan 28 20:00–24:00 UTC Jan 29 00:00–02:00, 20:00–24:00 UTC Jan 30 00:00–02:00, 20:00–24:00 UTC Jan 31 00:00–02:00, 21:00–24:00 UTC
February
Feb 1 00:00–01:00, 03:00–04:00 UTC Feb 2 22:00–24:00 UTC Feb 3 00:00–02:00 UTC Feb 4 08:00–09:00, 20:00–24:00 UTC Feb 5 00:00–02:00 UTC Feb 6 20:00–23:00 UTC Feb 7 00:00–03:00, 20:00–24:00 UTC Feb 8 00:00–02:00 UTC Feb 10 20:00–24:00 UTC Feb 11 00:00–03:00, 20:00–24:00 UTC Feb 12 00:00–03:00, 20:00–24:00 UTC Feb 13 00:00–03:00, 19:00–24:00 UTC Feb 14 00:00–02:00, 20:00–24:00 UTC Feb 15 00:00–03:00, 15:00–21:00 UTC Feb 16 23:00–24:00 UTC Feb 17 00:00–03:00, 21:00–24:00 UTC Feb 18 00:00–03:00, 21:00–24:00 UTC Feb 19 00:00–03:00 UTC Feb 20 21:00–24:00 UTC Feb 21 00:00–03:00, 21:00–24:00 UTC Feb 22 00:00–02:00, 21:00–24:00 UTC Feb 23 00:00–03:00 UTC Feb 27 21:00–24:00 UTC Feb 28 00:00–03:00, 20:00–24:00 UTC
March
Mar 1 00:00–03:00, 19:00–24:00 UTC Mar 2 00:00–02:00, 20:00–24:00 UTC Mar 3 00:00–01:00, 19:00–24:00 UTC Mar 4 00:00–01:00, 20:00–22:00 UTC Mar 5 01:00–02:00, 20:00–22:00 UTC Mar 6 00:00–02:00 UTC Mar 9 21:00–24:00 UTC Mar 10 00:00–02:00, 20:00–24:00 UTC Mar 11 00:00–02:00, 20:00–24:00 UTC Mar 12 00:00–03:00, 22:00–24:00 UTC Mar 13 00:00–03:00, 13:00–20:00 UTC Mar 14 20:00–24:00 UTC Mar 15 00:00–03:00, 19:00–20:00, 22:00–24:00 UTC Mar 16 00:00–02:00, 20:00–24:00 UTC Mar 17 00:00–02:00, 20:00–22:00 UTC Mar 18 20:00–24:00 UTC Mar 19 00:00–03:00, 23:00–24:00 UTC Mar 20 00:00–01:00, 16:00–23:00 UTC Mar 21 20:00–24:00 UTC Mar 22 00:00–02:00, 20:00–24:00 UTC Mar 23 00:00–02:00 UTC Mar 24 20:00–24:00 UTC Mar 25 00:00–03:00, 21:00–24:00 UTC Mar 26 00:00–03:00 UTC Mar 27 04:00–10:00, 23:00–24:00 UTC Mar 28 00:00–03:00, 20:00–24:00 UTC Mar 29 00:00–02:00, 21:00–24:00 UTC Mar 30 00:00–02:00, 20:00–24:00 UTC Mar 31 00:00–02:00, 22:00–23:00 UTC
April
Apr 1 00:00–02:00 UTC Apr 2 00:00–04:00, 19:00–23:00 UTC Apr 4 20:00–24:00 UTC Apr 5 00:00–03:00, 19:00–24:00 UTC Apr 6 00:00–02:00, 22:00–24:00 UTC Apr 7 00:00–01:00, 22:00–24:00 UTC Apr 8 00:00–03:00, 20:00–23:00 UTC Apr 9 00:00–02:00 UTC Apr 10 00:00–03:00 UTC Apr 11 21:00–24:00 UTC Apr 12 00:00–03:00, 20:00–24:00 UTC Apr 13 00:00–02:00, 21:00–24:00 UTC Apr 14 00:00–03:00, 20:00–24:00 UTC Apr 15 00:00–03:00, 20:00–23:00 UTC Apr 16 00:00–02:00, 21:00–24:00 UTC Apr 17 00:00–02:00 UTC Apr 18 20:00–24:00 UTC Apr 19 00:00–03:00 UTC Apr 20 21:00–24:00 UTC Apr 21 00:00–02:00, 20:00–22:00 UTC Apr 22 21:00–24:00 UTC Apr 23 00:00–03:00, 20:00–24:00 UTC Apr 24 00:00–01:00, 21:00–24:00 UTC Apr 25 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, 21:00–24:00 UTC Apr 30 00:00–03:00, 20:00–24:00 UTC
May
May 1 00:00–03:00, 13:00–19:00 UTC May 4 21:00–24:00 UTC May 5 00:00–03:00, 21:00–24:00 UTC May 6 00:00–03:00, 21:00–24:00 UTC May 7 00:00–03:00 UTC May 8 02:00–03:00, 21:00–24:00 UTC May 9 12:00–18:00 UTC May 11 21:00–24:00 UTC May 12 00:00–03:00, 20:00–24:00 UTC May 13 00:00–02:00, 21:00–24:00 UTC May 14 00:00–02:00 UTC May 15 21:00–23:00 UTC May 16 01:00–02:00 UTC May 18 20:00–24:00 UTC May 19 00:00–02:00, 20:00–24:00 UTC May 20 00:00–02:00, 20:00–22:00 UTC May 21 01:00–02:00, 21:00–24:00 UTC May 22 02:00–03:00 UTC May 25 21:00–24:00 UTC May 26 00:00–03:00, 21:00–24:00 UTC May 27 00:00–03:00, 20:00–24:00 UTC May 28 00:00–02:00, 21:00–24:00 UTC May 29 00:00–03:00 UTC May 30 21:00–24:00 UTC May 31 00:00–03:00 UTC
June
Jun 1 20:00–24:00 UTC Jun 2 00:00–02:00, 17:00–24:00 UTC Jun 3 18:00–24:00 UTC Jun 4 17:00–24:00 UTC Jun 5 16:00–22:00 UTC Jun 8 20:00–24:00 UTC Jun 9 00:00–03:00, 17:00–23:00 UTC Jun 11 18:00–23:00 UTC Jun 12 17:00–24:00 UTC Jun 13 17:00–19:00 UTC Jun 16 18:00–24:00 UTC Jun 30 20:00–24:00 UTC
July
Jul 1 00:00–02:00 UTC Jul 2 20:00–24:00 UTC Jul 3 00:00–02:00 UTC Jul 6 20:00–24:00 UTC Jul 7 00:00–02:00, 19:00–23:00 UTC Jul 8 20:00–24:00 UTC Jul 9 00:00–03:00 UTC Jul 13 20:00–24:00 UTC Jul 14 00:00–02:00, 20:00–24:00 UTC Jul 15 00:00–03:00, 21:00–24:00 UTC Jul 16 00:00–03:00 UTC Jul 21 21:00–24:00 UTC Jul 22 00:00–02:00 UTC