Most often in May, 26 days.
Highest follower count seen that year: 22,923.
January
Jan 1 10:00–17:00 UTC Jan 2 15:00–18:00 UTC Jan 7 21:00–24:00 UTC Jan 8 00:00–01:00 UTC Jan 9 05:00–12:00 UTC Jan 10 12:00–13:00, 15:00–23:00 UTC Jan 11 04:00–05:00, 10:00–16:00 UTC Jan 12 15:00–20:00 UTC Jan 13 13:00–19:00 UTC Jan 16 15:00–19:00 UTC Jan 17 15:00–23:00 UTC Jan 20 13:00–18:00 UTC Jan 21 14:00–18:00 UTC Jan 22 13:00–15:00, 17:00–18:00 UTC Jan 23 03:00–10:00 UTC Jan 24 14:00–20:00 UTC Jan 25 04:00–06:00, 09:00–14:00 UTC Jan 26 13:00–17:00 UTC Jan 27 10:00–17:00 UTC Jan 29 01:00–07:00, 09:00–12:00, 14:00–17:00 UTC Jan 30 13:00–18:00 UTC Jan 31 04:00–11:00 UTC
February
Feb 1 15:00–19:00 UTC Feb 2 10:00–13:00 UTC Feb 3 03:00–06:00 UTC Feb 4 04:00–05:00, 10:00–14:00 UTC Feb 6 20:00–23:00 UTC Feb 7 03:00–06:00, 16:00–18:00 UTC Feb 8 19:00–21:00 UTC Feb 9 03:00–06:00, 13:00–16:00 UTC Feb 11 04:00–13:00 UTC Feb 12 13:00–19:00 UTC Feb 13 17:00–19:00 UTC Feb 14 03:00–06:00, 17:00–19:00 UTC Feb 15 12:00–14:00, 23:00–24:00 UTC Feb 16 00:00–01:00 UTC Feb 19 14:00–18:00, 20:00–23:00 UTC Feb 20 01:00–06:00, 09:00–11:00 UTC Feb 23 15:00–18:00 UTC Feb 26 03:00–07:00, 15:00–18:00 UTC Feb 27 16:00–18:00 UTC Feb 28 20:00–24:00 UTC
March
Mar 1 00:00–01:00, 17:00–19:00, 23:00–24:00 UTC Mar 2 00:00–01:00, 15:00–17:00 UTC Mar 3 10:00–12:00 UTC Mar 5 02:00–09:00 UTC Mar 6 08:00–09:00 UTC Mar 7 15:00–19:00 UTC Mar 8 20:00–23:00 UTC Mar 9 04:00–11:00 UTC Mar 10 04:00–09:00 UTC Mar 11 04:00–09:00 UTC Mar 12 04:00–10:00 UTC Mar 16 04:00–08:00 UTC Mar 19 05:00–09:00 UTC Mar 20 04:00–08:00 UTC Mar 21 16:00–21:00 UTC Mar 22 19:00–24:00 UTC Mar 23 16:00–19:00 UTC Mar 24 02:00–05:00 UTC Mar 26 02:00–06:00, 10:00–12:00 UTC Mar 28 22:00–24:00 UTC Mar 29 00:00–07:00, 17:00–20:00 UTC Mar 30 02:00–09:00 UTC Mar 31 03:00–08:00 UTC
April
Apr 2 00:00–07:00, 22:00–24:00 UTC Apr 3 00:00–02:00 UTC Apr 4 03:00–09:00, 17:00–20:00 UTC Apr 5 04:00–07:00, 18:00–20:00 UTC Apr 6 03:00–08:00 UTC Apr 7 04:00–08:00 UTC Apr 8 22:00–23:00 UTC Apr 9 02:00–06:00, 13:00–17:00 UTC Apr 10 04:00–06:00 UTC Apr 11 03:00–08:00 UTC Apr 12 04:00–07:00, 18:00–20:00 UTC Apr 14 20:00–23:00 UTC Apr 15 03:00–06:00 UTC Apr 16 10:00–14:00, 16:00–19:00 UTC Apr 17 15:00–22:00 UTC Apr 18 04:00–07:00 UTC Apr 19 14:00–18:00 UTC Apr 20 03:00–06:00 UTC Apr 23 02:00–06:00, 11:00–14:00 UTC Apr 24 09:00–12:00, 15:00–19:00 UTC Apr 25 17:00–22:00 UTC Apr 26 03:00–06:00, 14:00–18:00 UTC Apr 28 03:00–05:00 UTC Apr 29 03:00–06:00, 22:00–24:00 UTC Apr 30 00:00–07:00 UTC
May
May 1 22:00–24:00 UTC May 2 00:00–07:00 UTC May 4 05:00–06:00 UTC May 5 03:00–06:00 UTC May 7 14:00–18:00 UTC May 8 05:00–10:00 UTC May 9 21:00–24:00 UTC May 10 06:00–10:00, 22:00–24:00 UTC May 11 04:00–06:00 UTC May 12 03:00–09:00 UTC May 15 05:00–10:00, 20:00–23:00 UTC May 16 04:00–05:00 UTC May 17 05:00–11:00, 23:00–24:00 UTC May 18 00:00–01:00, 04:00–07:00 UTC May 19 05:00–07:00 UTC May 21 04:00–08:00 UTC May 22 19:00–20:00, 22:00–24:00 UTC May 23 04:00–09:00 UTC May 24 20:00–24:00 UTC May 25 00:00–05:00 UTC May 26 03:00–06:00 UTC May 27 04:00–07:00 UTC May 28 04:00–08:00, 17:00–19:00 UTC May 29 05:00–08:00 UTC May 30 22:00–24:00 UTC May 31 00:00–02:00, 04:00–09:00 UTC
June
Jun 1 11:00–14:00, 16:00–19:00 UTC Jun 2 16:00–19:00 UTC Jun 3 15:00–17:00 UTC Jun 4 04:00–07:00 UTC Jun 6 04:00–07:00 UTC Jun 7 22:00–24:00 UTC Jun 8 00:00–02:00 UTC Jun 9 15:00–17:00 UTC Jun 10 05:00–08:00, 17:00–18:00 UTC Jun 11 17:00–18:00, 21:00–23:00 UTC Jun 12 05:00–07:00 UTC Jun 13 05:00–13:00 UTC Jun 16 02:00–05:00 UTC Jun 17 05:00–08:00 UTC Jun 18 03:00–09:00 UTC Jun 19 16:00–19:00 UTC Jun 20 04:00–07:00 UTC Jun 21 03:00–06:00 UTC Jun 24 02:00–06:00 UTC Jun 26 17:00–20:00 UTC Jun 27 04:00–07:00, 15:00–17:00, 20:00–22:00 UTC
July
Jul 1 17:00–19:00 UTC Jul 2 04:00–06:00 UTC Jul 3 04:00–08:00, 17:00–19:00 UTC Jul 4 19:00–23:00 UTC Jul 5 00:00–02:00, 05:00–06:00, 19:00–21:00 UTC Jul 7 04:00–07:00 UTC Jul 8 15:00–16:00, 20:00–22:00 UTC Jul 9 04:00–06:00 UTC Jul 10 04:00–07:00, 19:00–22:00 UTC Jul 11 04:00–08:00 UTC Jul 12 19:00–24:00 UTC Jul 13 04:00–06:00 UTC Jul 14 20:00–22:00 UTC Jul 15 20:00–22:00 UTC Jul 16 20:00–23:00 UTC Jul 17 04:00–08:00 UTC Jul 18 17:00–19:00, 21:00–24:00 UTC Jul 19 03:00–09:00, 19:00–23:00 UTC Jul 21 20:00–23:00 UTC Jul 22 21:00–22:00 UTC Jul 23 05:00–11:00 UTC Jul 24 05:00–07:00, 17:00–18:00, 20:00–24:00 UTC Jul 25 00:00–01:00, 03:00–06:00 UTC Jul 26 16:00–17:00 UTC