Most often in April, 29 days.
Highest follower count seen that year: 6,947.
January
Jan 1 22:00–24:00 UTC Jan 2 00:00–08:00 UTC Jan 5 00:00–07:00, 23:00–24:00 UTC Jan 6 00:00–05:00, 07:00–15:00 UTC Jan 8 22:00–24:00 UTC Jan 9 00:00–08:00 UTC Jan 11 23:00–24:00 UTC Jan 12 00:00–07:00 UTC Jan 13 01:00–08:00 UTC Jan 15 01:00–04:00, 06:00–09:00 UTC Jan 17 23:00–24:00 UTC Jan 18 00:00–08:00 UTC Jan 19 00:00–06:00 UTC Jan 21 01:00–08:00, 22:00–24:00 UTC Jan 22 00:00–07:00 UTC Jan 23 08:00–14:00 UTC Jan 24 21:00–24:00 UTC Jan 25 00:00–07:00, 16:00–20:00, 22:00–24:00 UTC Jan 26 03:00–08:00 UTC Jan 27 15:00–23:00 UTC Jan 28 00:00–01:00 UTC Jan 29 00:00–08:00 UTC Jan 30 16:00–22:00 UTC Jan 31 17:00–21:00 UTC
February
Feb 4 23:00–24:00 UTC Feb 5 00:00–08:00 UTC Feb 6 00:00–08:00 UTC Feb 8 00:00–07:00 UTC Feb 11 00:00–02:00, 04:00–08:00 UTC Feb 12 02:00–07:00, 12:00–13:00 UTC Feb 14 00:00–08:00, 23:00–24:00 UTC Feb 15 00:00–08:00, 17:00–21:00, 23:00–24:00 UTC Feb 16 01:00–07:00 UTC Feb 17 21:00–24:00 UTC Feb 18 00:00–06:00 UTC Feb 20 23:00–24:00 UTC Feb 21 00:00–07:00 UTC Feb 22 22:00–24:00 UTC Feb 23 00:00–07:00 UTC Feb 24 03:00–08:00 UTC Feb 25 23:00–24:00 UTC Feb 26 00:00–09:00 UTC
March
Mar 1 00:00–09:00, 23:00–24:00 UTC Mar 2 00:00–06:00 UTC Mar 3 23:00–24:00 UTC Mar 4 00:00–06:00 UTC Mar 5 01:00–06:00 UTC Mar 6 14:00–19:00, 21:00–24:00 UTC Mar 7 00:00–04:00, 06:00–07:00 UTC Mar 8 15:00–24:00 UTC Mar 9 00:00–02:00, 05:00–06:00 UTC Mar 11 01:00–08:00, 10:00–13:00 UTC Mar 12 00:00–07:00 UTC Mar 13 15:00–24:00 UTC Mar 14 00:00–07:00 UTC Mar 15 16:00–20:00, 23:00–24:00 UTC Mar 16 00:00–04:00 UTC Mar 17 16:00–19:00 UTC Mar 18 01:00–03:00 UTC Mar 24 15:00–18:00, 20:00–24:00 UTC Mar 25 00:00–07:00 UTC Mar 27 22:00–24:00 UTC Mar 28 01:00–06:00 UTC Mar 29 17:00–24:00 UTC Mar 30 00:00–02:00 UTC Mar 31 15:00–18:00, 20:00–21:00, 23:00–24:00 UTC
April
Apr 1 00:00–04:00 UTC Apr 3 15:00–17:00 UTC Apr 4 02:00–06:00 UTC Apr 5 14:00–18:00, 20:00–24:00 UTC Apr 6 01:00–06:00 UTC Apr 7 14:00–16:00, 19:00–20:00, 22:00–24:00 UTC Apr 8 00:00–06:00 UTC Apr 9 16:00–17:00 UTC Apr 10 01:00–06:00 UTC Apr 11 04:00–06:00 UTC Apr 12 15:00–24:00 UTC Apr 13 00:00–05:00 UTC Apr 14 23:00–24:00 UTC Apr 15 00:00–05:00 UTC Apr 16 22:00–24:00 UTC Apr 17 00:00–02:00, 04:00–05:00, 22:00–24:00 UTC Apr 18 00:00–06:00 UTC Apr 19 23:00–24:00 UTC Apr 20 00:00–05:00 UTC Apr 21 22:00–24:00 UTC Apr 22 00:00–05:00 UTC Apr 23 22:00–24:00 UTC Apr 24 00:00–01:00, 03:00–05:00 UTC Apr 25 21:00–24:00 UTC Apr 26 00:00–06:00 UTC Apr 27 01:00–07:00 UTC Apr 28 23:00–24:00 UTC Apr 29 00:00–06:00 UTC Apr 30 01:00–07:00, 22:00–24:00 UTC
May
May 1 00:00–05:00 UTC May 3 00:00–07:00 UTC May 6 00:00–13:00 UTC May 8 22:00–24:00 UTC May 9 00:00–01:00, 03:00–06:00, 22:00–24:00 UTC May 10 00:00–06:00, 22:00–24:00 UTC May 11 00:00–06:00 UTC May 13 01:00–06:00 UTC May 15 02:00–06:00, 22:00–23:00 UTC May 16 03:00–06:00 UTC May 17 22:00–24:00 UTC May 18 00:00–06:00 UTC May 19 23:00–24:00 UTC May 20 00:00–02:00, 04:00–06:00 UTC May 21 22:00–24:00 UTC May 22 00:00–04:00, 20:00–22:00 UTC May 23 00:00–06:00, 22:00–24:00 UTC May 24 00:00–06:00, 21:00–24:00 UTC May 25 00:00–01:00, 03:00–06:00 UTC May 26 22:00–24:00 UTC May 27 00:00–05:00 UTC May 28 21:00–24:00 UTC May 29 00:00–01:00, 03:00–06:00, 21:00–24:00 UTC May 30 00:00–05:00, 22:00–23:00 UTC May 31 02:00–04:00, 22:00–24:00 UTC
June
Jun 1 00:00–01:00, 03:00–05:00 UTC Jun 5 02:00–06:00 UTC Jun 6 00:00–07:00 UTC Jun 7 01:00–07:00 UTC Jun 8 01:00–06:00 UTC Jun 11 22:00–24:00 UTC Jun 12 00:00–08:00 UTC Jun 13 00:00–08:00 UTC Jun 15 03:00–06:00 UTC Jun 16 00:00–06:00 UTC Jun 19 19:00–22:00 UTC Jun 20 01:00–05:00, 19:00–24:00 UTC Jun 21 00:00–02:00, 05:00–06:00, 20:00–23:00 UTC Jun 23 23:00–24:00 UTC Jun 24 00:00–06:00 UTC Jun 26 00:00–07:00 UTC Jun 27 00:00–06:00, 22:00–24:00 UTC Jun 28 00:00–07:00, 23:00–24:00 UTC Jun 29 00:00–06:00, 23:00–24:00 UTC Jun 30 00:00–06:00, 22:00–24:00 UTC
July
Jul 1 00:00–05:00 UTC Jul 4 20:00–23:00 UTC Jul 5 00:00–07:00, 20:00–23:00 UTC Jul 6 03:00–06:00 UTC Jul 7 22:00–24:00 UTC Jul 8 00:00–02:00, 04:00–07:00, 23:00–24:00 UTC Jul 9 00:00–06:00 UTC Jul 11 20:00–24:00 UTC Jul 12 03:00–11:00, 21:00–24:00 UTC Jul 13 00:00–04:00 UTC Jul 15 00:00–02:00, 04:00–05:00 UTC Jul 16 01:00–05:00 UTC Jul 18 00:00–08:00, 23:00–24:00 UTC Jul 19 00:00–12:00 UTC Jul 20 00:00–11:00 UTC Jul 24 00:00–06:00, 23:00–24:00 UTC Jul 25 01:00–05:00, 23:00–24:00 UTC Jul 26 01:00–04:00 UTC Jul 27 01:00–02:00, 05:00–06:00 UTC