Most often in June, 29 days.
Highest follower count seen that year: 17,871.
January
Jan 1 02:00–05:00, 21:00–24:00 UTC Jan 2 00:00–01:00, 04:00–05:00 UTC Jan 3 23:00–24:00 UTC Jan 4 00:00–02:00, 04:00–06:00 UTC Jan 5 00:00–03:00 UTC Jan 9 02:00–09:00 UTC Jan 10 00:00–04:00 UTC Jan 11 21:00–24:00 UTC Jan 12 00:00–02:00, 23:00–24:00 UTC Jan 13 00:00–05:00 UTC Jan 14 01:00–04:00 UTC Jan 15 00:00–04:00, 22:00–24:00 UTC Jan 16 00:00–05:00 UTC Jan 17 03:00–05:00 UTC Jan 19 00:00–03:00 UTC Jan 20 02:00–04:00 UTC Jan 21 23:00–24:00 UTC Jan 22 00:00–04:00, 21:00–24:00 UTC Jan 23 00:00–05:00 UTC Jan 24 00:00–03:00, 20:00–24:00 UTC Jan 25 00:00–01:00, 03:00–05:00, 22:00–24:00 UTC Jan 26 00:00–01:00, 23:00–24:00 UTC Jan 27 00:00–02:00 UTC Jan 28 20:00–24:00 UTC Jan 29 00:00–01:00 UTC Jan 30 00:00–04:00 UTC Jan 31 17:00–21:00 UTC
February
Feb 1 22:00–24:00 UTC Feb 2 00:00–01:00 UTC Feb 3 01:00–04:00, 06:00–07:00 UTC Feb 4 01:00–04:00, 16:00–23:00 UTC Feb 5 02:00–05:00 UTC Feb 6 03:00–06:00 UTC Feb 7 03:00–04:00 UTC Feb 8 02:00–04:00, 23:00–24:00 UTC Feb 9 00:00–03:00 UTC Feb 10 01:00–05:00 UTC Feb 12 00:00–06:00 UTC Feb 13 00:00–04:00, 22:00–24:00 UTC Feb 14 00:00–07:00, 20:00–22:00 UTC Feb 15 02:00–03:00, 22:00–24:00 UTC Feb 16 00:00–05:00 UTC Feb 17 00:00–05:00 UTC Feb 18 01:00–02:00 UTC Feb 19 00:00–05:00, 21:00–24:00 UTC Feb 20 00:00–06:00, 23:00–24:00 UTC Feb 21 00:00–06:00 UTC Feb 22 02:00–05:00 UTC Feb 23 22:00–24:00 UTC Feb 24 00:00–02:00, 22:00–24:00 UTC Feb 25 00:00–01:00 UTC Feb 26 00:00–05:00, 13:00–15:00 UTC Feb 28 03:00–09:00 UTC
March
Mar 1 05:00–09:00, 22:00–24:00 UTC Mar 2 00:00–04:00 UTC Mar 3 22:00–24:00 UTC Mar 4 00:00–04:00 UTC Mar 5 00:00–04:00 UTC Mar 6 00:00–05:00 UTC Mar 7 01:00–02:00 UTC Mar 8 02:00–09:00, 23:00–24:00 UTC Mar 9 00:00–03:00 UTC Mar 12 02:00–05:00 UTC Mar 15 03:00–06:00 UTC Mar 16 01:00–03:00 UTC Mar 18 23:00–24:00 UTC Mar 19 00:00–05:00 UTC Mar 20 00:00–05:00 UTC Mar 22 01:00–02:00, 04:00–06:00, 21:00–24:00 UTC Mar 23 00:00–06:00 UTC Mar 24 17:00–21:00 UTC Mar 25 03:00–04:00, 17:00–20:00 UTC Mar 26 17:00–21:00 UTC Mar 27 18:00–21:00 UTC Mar 29 03:00–06:00, 19:00–24:00 UTC Mar 30 02:00–06:00 UTC Mar 31 19:00–21:00 UTC
April
Apr 1 17:00–20:00 UTC Apr 4 04:00–08:00 UTC Apr 5 22:00–24:00 UTC Apr 6 00:00–04:00 UTC Apr 8 16:00–20:00 UTC Apr 10 01:00–03:00 UTC Apr 12 22:00–24:00 UTC Apr 13 00:00–01:00, 19:00–21:00 UTC Apr 14 02:00–05:00, 20:00–21:00 UTC Apr 16 02:00–03:00 UTC Apr 17 19:00–21:00 UTC Apr 18 03:00–06:00 UTC Apr 20 16:00–19:00 UTC Apr 21 21:00–22:00 UTC Apr 22 03:00–05:00 UTC Apr 24 03:00–06:00, 18:00–20:00 UTC Apr 27 00:00–02:00, 18:00–20:00 UTC Apr 28 20:00–23:00 UTC Apr 29 01:00–03:00 UTC Apr 30 20:00–22:00 UTC
May
May 1 04:00–06:00 UTC May 2 02:00–06:00 UTC May 3 22:00–24:00 UTC May 4 00:00–01:00, 05:00–07:00 UTC May 5 20:00–22:00 UTC May 6 21:00–22:00 UTC May 7 19:00–20:00 UTC May 8 20:00–22:00 UTC May 10 00:00–06:00 UTC May 11 19:00–20:00 UTC May 12 15:00–17:00 UTC May 13 03:00–05:00, 18:00–20:00 UTC May 14 19:00–22:00 UTC May 15 02:00–06:00 UTC May 17 04:00–06:00, 19:00–23:00 UTC May 18 02:00–05:00 UTC May 19 19:00–24:00 UTC May 20 00:00–03:00 UTC May 21 17:00–20:00 UTC May 22 03:00–05:00, 19:00–22:00 UTC May 24 23:00–24:00 UTC May 25 00:00–08:00, 14:00–16:00, 18:00–20:00 UTC May 27 20:00–23:00 UTC May 28 02:00–05:00, 18:00–22:00 UTC May 31 22:00–24:00 UTC
June
Jun 1 00:00–05:00 UTC Jun 2 19:00–20:00 UTC Jun 3 02:00–05:00, 19:00–24:00 UTC Jun 4 00:00–04:00, 21:00–23:00 UTC Jun 5 00:00–01:00, 22:00–24:00 UTC Jun 6 01:00–02:00 UTC Jun 7 03:00–06:00, 23:00–24:00 UTC Jun 8 01:00–04:00 UTC Jun 9 20:00–22:00 UTC Jun 10 22:00–24:00 UTC Jun 11 00:00–05:00, 15:00–19:00 UTC Jun 12 01:00–04:00, 19:00–20:00 UTC Jun 13 05:00–07:00 UTC Jun 15 01:00–06:00, 19:00–23:00 UTC Jun 16 01:00–02:00, 17:00–19:00 UTC Jun 17 03:00–05:00, 18:00–22:00 UTC Jun 18 19:00–20:00 UTC Jun 19 20:00–22:00 UTC Jun 20 03:00–05:00 UTC Jun 21 03:00–07:00, 09:00–12:00, 19:00–24:00 UTC Jun 22 17:00–22:00 UTC Jun 23 18:00–20:00 UTC Jun 24 00:00–02:00, 18:00–20:00 UTC Jun 25 16:00–20:00 UTC Jun 26 04:00–05:00 UTC Jun 27 05:00–06:00, 17:00–19:00 UTC Jun 28 22:00–24:00 UTC Jun 29 00:00–07:00, 22:00–24:00 UTC Jun 30 00:00–04:00, 18:00–20:00 UTC
July
Jul 1 01:00–05:00, 19:00–23:00 UTC Jul 2 02:00–07:00, 18:00–20:00 UTC Jul 3 18:00–22:00 UTC Jul 5 21:00–24:00 UTC Jul 6 00:00–05:00, 18:00–23:00 UTC Jul 7 18:00–20:00 UTC Jul 8 18:00–22:00 UTC Jul 9 02:00–05:00, 19:00–22:00 UTC Jul 10 19:00–20:00 UTC Jul 11 18:00–20:00 UTC Jul 12 05:00–08:00, 21:00–24:00 UTC Jul 13 00:00–05:00 UTC Jul 14 03:00–06:00, 18:00–19:00 UTC Jul 15 15:00–21:00 UTC Jul 16 22:00–24:00 UTC Jul 18 05:00–07:00 UTC Jul 19 02:00–08:00, 22:00–24:00 UTC Jul 20 00:00–05:00 UTC Jul 21 20:00–21:00 UTC Jul 22 03:00–05:00, 21:00–23:00 UTC Jul 23 04:00–07:00 UTC Jul 25 04:00–05:00 UTC Jul 26 00:00–02:00, 23:00–24:00 UTC Jul 27 19:00–21:00 UTC