Most often in March, 30 days.
Highest follower count seen that year: 6,735.
January
Jan 1 00:00–01:00, 20:00–24:00 UTC Jan 2 00:00–02:00, 20:00–24:00 UTC Jan 3 21:00–24:00 UTC Jan 4 00:00–01:00 UTC Jan 6 20:00–24:00 UTC Jan 7 22:00–24:00 UTC Jan 8 18:00–24:00 UTC Jan 9 19:00–24:00 UTC Jan 10 22:00–24:00 UTC Jan 11 00:00–02:00, 21:00–24:00 UTC Jan 12 00:00–01:00, 21:00–24:00 UTC Jan 14 18:00–24:00 UTC Jan 15 21:00–24:00 UTC Jan 17 22:00–24:00 UTC Jan 18 00:00–02:00 UTC Jan 19 20:00–24:00 UTC Jan 20 00:00–01:00 UTC Jan 21 18:00–19:00, 21:00–24:00 UTC Jan 22 00:00–02:00, 19:00–24:00 UTC Jan 23 00:00–01:00, 21:00–24:00 UTC Jan 24 00:00–01:00, 22:00–24:00 UTC Jan 25 01:00–02:00, 22:00–24:00 UTC Jan 26 00:00–01:00, 20:00–24:00 UTC Jan 27 00:00–01:00, 22:00–23:00 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, 21:00–24:00 UTC Jan 31 00:00–01:00, 21:00–24:00 UTC
February
Feb 1 00:00–01:00 UTC Feb 3 16:00–24:00 UTC Feb 4 17:00–19:00, 22:00–24:00 UTC Feb 5 00:00–01:00 UTC Feb 6 18:00–19:00, 21:00–24:00 UTC Feb 7 00:00–02:00, 21:00–24:00 UTC Feb 8 00:00–02:00, 19:00–24:00 UTC Feb 9 00:00–02:00, 20:00–24:00 UTC Feb 10 00:00–02:00, 17:00–21:00, 23:00–24:00 UTC Feb 11 00:00–03:00, 20:00–24:00 UTC Feb 12 00:00–03:00, 19:00–24:00 UTC Feb 13 00:00–02:00, 17:00–22:00 UTC Feb 14 01:00–06:00, 13:00–14:00 UTC Feb 15 21:00–24:00 UTC Feb 16 00:00–07:00 UTC Feb 17 01:00–08:00 UTC Feb 18 21:00–22:00 UTC Feb 19 01:00–09:00 UTC Feb 21 04:00–06:00, 15:00–16:00, 18:00–23:00 UTC Feb 22 00:00–03:00, 06:00–10:00, 17:00–18:00, 20:00–24:00 UTC Feb 23 02:00–07:00, 15:00–17:00, 20:00–24:00 UTC Feb 24 00:00–01:00, 05:00–07:00 UTC Feb 25 17:00–23:00 UTC Feb 26 00:00–01:00, 06:00–10:00, 20:00–21:00, 23:00–24:00 UTC Feb 27 00:00–06:00 UTC Feb 28 07:00–09:00 UTC
March
Mar 1 01:00–10:00, 18:00–20:00 UTC Mar 2 21:00–23:00 UTC Mar 3 00:00–01:00, 18:00–24:00 UTC Mar 4 00:00–04:00, 08:00–10:00, 20:00–22:00 UTC Mar 5 00:00–02:00 UTC Mar 6 00:00–03:00, 18:00–20:00 UTC Mar 7 00:00–01:00, 22:00–24:00 UTC Mar 8 00:00–02:00, 07:00–09:00 UTC Mar 9 05:00–06:00, 15:00–24:00 UTC Mar 10 00:00–01:00, 19:00–24:00 UTC Mar 11 20:00–24:00 UTC Mar 12 00:00–03:00, 16:00–21:00 UTC Mar 13 19:00–24:00 UTC Mar 14 00:00–01:00, 21:00–24:00 UTC Mar 15 00:00–01:00, 17:00–22:00 UTC Mar 16 19:00–20:00, 22:00–24:00 UTC Mar 17 20:00–24:00 UTC Mar 18 01:00–02:00, 20:00–24:00 UTC Mar 19 00:00–01:00, 15:00–19:00 UTC Mar 20 19:00–24:00 UTC Mar 21 00:00–01:00, 21:00–24:00 UTC Mar 23 22:00–24:00 UTC Mar 24 00:00–07:00, 18:00–23:00 UTC Mar 25 21:00–24:00 UTC Mar 26 00:00–02:00, 20:00–24:00 UTC Mar 27 00:00–02:00, 18:00–22:00 UTC Mar 28 00:00–01:00, 19:00–23:00 UTC Mar 29 23:00–24:00 UTC Mar 30 00:00–03:00, 21:00–24:00 UTC Mar 31 19:00–24:00 UTC
April
Apr 1 00:00–01:00, 20:00–24:00 UTC Apr 2 00:00–01:00 UTC Apr 4 23:00–24:00 UTC Apr 5 00:00–07:00, 12:00–14:00, 18:00–24:00 UTC Apr 6 04:00–07:00, 20:00–24:00 UTC Apr 8 22:00–23:00 UTC Apr 9 00:00–01:00 UTC Apr 10 21:00–22:00 UTC Apr 11 21:00–24:00 UTC Apr 14 22:00–23:00 UTC Apr 15 00:00–01:00, 20:00–24:00 UTC Apr 19 22:00–24:00 UTC Apr 20 00:00–03:00, 20:00–24:00 UTC Apr 21 00:00–02:00 UTC Apr 23 21:00–24:00 UTC Apr 24 00:00–02:00, 20:00–22:00 UTC Apr 25 00:00–01:00, 20:00–21:00 UTC Apr 26 14:00–15:00 UTC Apr 27 21:00–24:00 UTC Apr 28 00:00–01:00, 21:00–24:00 UTC Apr 29 00:00–01:00 UTC Apr 30 19:00–21:00 UTC
May
May 1 00:00–01:00 UTC May 4 20:00–24:00 UTC May 5 00:00–01:00, 16:00–17:00, 19:00–23:00 UTC May 6 16:00–17:00, 20:00–22:00 UTC May 7 17:00–21:00 UTC May 8 16:00–20:00 UTC May 9 16:00–19:00 UTC May 10 16:00–22:00 UTC May 11 16:00–22:00 UTC May 12 17:00–24:00 UTC May 13 17:00–23:00 UTC May 14 16:00–17:00, 19:00–20:00 UTC May 15 15:00–20:00 UTC May 16 15:00–17:00 UTC May 17 15:00–16:00, 19:00–22:00 UTC May 19 16:00–17:00 UTC May 20 15:00–19:00 UTC May 21 16:00–21:00 UTC May 22 15:00–19:00 UTC May 27 16:00–20:00, 22:00–23:00 UTC May 29 16:00–23:00 UTC May 30 17:00–23:00 UTC
June
Jun 1 19:00–22:00 UTC Jun 3 18:00–23:00 UTC Jun 4 19:00–24:00 UTC Jun 5 15:00–19:00 UTC Jun 6 15:00–20:00 UTC Jun 7 15:00–19:00 UTC Jun 9 16:00–22:00 UTC Jun 10 16:00–17:00, 19:00–23:00 UTC Jun 11 15:00–18:00 UTC Jun 12 16:00–22:00 UTC Jun 13 17:00–19:00 UTC Jun 15 15:00–23:00 UTC Jun 17 16:00–21:00 UTC Jun 18 15:00–22:00 UTC Jun 19 15:00–21:00 UTC Jun 20 16:00–22:00 UTC Jun 21 14:00–17:00, 19:00–20:00 UTC Jun 23 14:00–15:00, 17:00–19:00 UTC Jun 24 15:00–16:00, 18:00–21:00 UTC Jun 25 14:00–22:00 UTC Jun 26 17:00–19:00 UTC Jun 28 17:00–19:00 UTC Jun 29 15:00–22:00 UTC
July
Jul 4 13:00–19:00, 21:00–22:00 UTC Jul 5 02:00–04:00, 15:00–18:00, 20:00–21:00 UTC Jul 6 15:00–16:00, 20:00–22:00 UTC Jul 7 19:00–22:00 UTC Jul 8 16:00–20:00 UTC Jul 9 16:00–21:00 UTC Jul 10 18:00–21:00 UTC Jul 11 16:00–22:00 UTC Jul 12 15:00–21:00 UTC Jul 13 13:00–17:00 UTC Jul 14 17:00–21:00 UTC Jul 15 16:00–22:00 UTC Jul 16 15:00–22:00 UTC Jul 17 15:00–19:00 UTC Jul 18 16:00–17:00, 19:00–20:00 UTC Jul 19 14:00–16:00, 19:00–20:00 UTC Jul 20 13:00–16:00, 19:00–24:00 UTC Jul 21 16:00–21:00 UTC Jul 23 18:00–21:00, 23:00–24:00 UTC Jul 24 00:00–01:00, 16:00–21:00 UTC Jul 26 15:00–23:00 UTC Jul 27 00:00–02:00 UTC