Most often in June, 29 days.
Highest follower count seen that year: 4,691.
January
Jan 4 20:00–24:00 UTC Jan 5 00:00–01:00, 22:00–24:00 UTC Jan 6 00:00–01:00 UTC Jan 9 21:00–24:00 UTC Jan 10 00:00–02:00 UTC Jan 11 22:00–24:00 UTC Jan 12 00:00–01:00 UTC Jan 13 22:00–24:00 UTC Jan 14 00:00–02:00 UTC Jan 17 22:00–24:00 UTC Jan 18 00:00–02:00 UTC Jan 27 22:00–24:00 UTC Jan 28 00:00–01:00, 18:00–24:00 UTC
February
Feb 7 00:00–02:00 UTC Feb 10 22:00–24:00 UTC Feb 11 00:00–01:00, 22:00–24:00 UTC Feb 12 00:00–01:00, 23:00–24:00 UTC Feb 13 00:00–01:00, 22:00–24:00 UTC Feb 14 00:00–01:00, 22:00–24:00 UTC Feb 15 00:00–03:00, 15:00–18:00 UTC Feb 16 00:00–01:00, 21:00–24:00 UTC Feb 17 00:00–01:00 UTC Feb 23 22:00–24:00 UTC Feb 24 00:00–02:00, 22:00–24:00 UTC Feb 25 00:00–02:00 UTC Feb 27 23:00–24:00 UTC Feb 28 00:00–02:00 UTC
March
Mar 1 00:00–02:00 UTC Mar 7 00:00–02:00 UTC Mar 19 20:00–23:00 UTC Mar 20 14:00–16:00, 20:00–23:00 UTC Mar 21 14:00–16:00 UTC Mar 24 15:00–17:00, 19:00–23:00 UTC Mar 25 14:00–18:00 UTC Mar 27 15:00–17:00, 19:00–21:00 UTC Mar 28 15:00–16:00, 21:00–22:00 UTC Mar 31 15:00–16:00 UTC
April
Apr 1 14:00–15:00 UTC Apr 4 13:00–15:00 UTC Apr 6 19:00–22:00 UTC Apr 8 15:00–17:00 UTC Apr 9 14:00–17:00 UTC Apr 10 14:00–17:00 UTC Apr 11 14:00–17:00, 22:00–24:00 UTC Apr 12 00:00–01:00 UTC Apr 13 14:00–17:00, 19:00–20:00, 22:00–24:00 UTC
May
May 22 14:00–16:00 UTC May 23 23:00–24:00 UTC May 24 00:00–03:00, 15:00–24:00 UTC May 25 00:00–03:00, 20:00–24:00 UTC May 26 00:00–04:00, 20:00–24:00 UTC May 27 00:00–03:00, 22:00–24:00 UTC May 28 00:00–03:00, 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 00:00–03:00, 21:00–24:00 UTC
June
Jun 1 00:00–03:00 UTC Jun 2 20:00–24:00 UTC Jun 3 00:00–03:00, 20:00–24:00 UTC Jun 4 00:00–03:00, 13:00–17:00 UTC Jun 5 20:00–24:00 UTC Jun 6 00:00–04:00, 20:00–24:00 UTC Jun 7 00:00–04:00, 21:00–23:00 UTC Jun 8 00:00–04:00, 20:00–24:00 UTC Jun 9 00:00–03:00, 21:00–24:00 UTC Jun 10 00:00–10:00, 21:00–24:00 UTC Jun 11 00:00–03:00, 20:00–24:00 UTC Jun 12 00:00–03:00, 05:00–09:00, 17:00–24:00 UTC Jun 13 00:00–03:00 UTC Jun 15 02:00–03:00, 05:00–08:00, 20:00–24:00 UTC Jun 16 00:00–03:00 UTC Jun 17 20:00–24:00 UTC Jun 18 01:00–03:00, 19:00–24:00 UTC Jun 19 00:00–03:00 UTC Jun 20 17:00–24:00 UTC Jun 21 00:00–05:00, 19:00–24:00 UTC Jun 22 00:00–06:00, 16:00–18:00, 20:00–24:00 UTC Jun 23 00:00–01:00, 04:00–06:00, 15:00–24:00 UTC Jun 24 00:00–03:00 UTC Jun 25 16:00–18:00, 20:00–24:00 UTC Jun 26 00:00–03:00, 20:00–24:00 UTC Jun 27 00:00–06:00, 16:00–24:00 UTC Jun 28 00:00–05:00, 19:00–24:00 UTC Jun 29 00:00–05:00, 20:00–24:00 UTC Jun 30 00:00–05:00, 20:00–24:00 UTC
July
Jul 1 00:00–03:00, 20:00–24:00 UTC Jul 2 00:00–03:00, 20:00–24:00 UTC Jul 3 00:00–04:00 UTC Jul 4 19:00–24:00 UTC Jul 5 00:00–07:00, 22:00–24:00 UTC Jul 6 00:00–01:00, 04:00–05:00, 20:00–24:00 UTC Jul 7 00:00–06:00, 22:00–24:00 UTC Jul 8 00:00–07:00, 20:00–24:00 UTC Jul 9 00:00–07:00, 20:00–24:00 UTC Jul 10 00:00–04:00, 21:00–24:00 UTC Jul 11 00:00–07:00, 20:00–24:00 UTC Jul 12 00:00–03:00, 12:00–17:00, 19:00–24:00 UTC Jul 13 00:00–03:00, 12:00–18:00, 20:00–24:00 UTC Jul 14 00:00–04:00, 14:00–19:00 UTC Jul 15 00:00–03:00, 21:00–24:00 UTC Jul 16 00:00–04:00 UTC Jul 17 16:00–24:00 UTC Jul 18 00:00–04:00, 15:00–18:00, 20:00–24:00 UTC Jul 19 00:00–03:00 UTC Jul 20 22:00–24:00 UTC Jul 21 00:00–02:00, 15:00–18:00, 20:00–24:00 UTC Jul 22 00:00–01:00, 14:00–19:00, 21:00–24:00 UTC Jul 23 00:00–05:00, 23:00–24:00 UTC Jul 24 00:00–04:00, 21:00–24:00 UTC Jul 25 00:00–02:00, 16:00–24:00 UTC Jul 26 00:00–02:00, 13:00–17:00, 20:00–24:00 UTC Jul 27 00:00–02:00 UTC