Most often in January, 31 days.
Highest follower count seen that year: 36,838.
January
Jan 1 00:00–02:00, 09:00–11:00, 14:00–19:00 UTC Jan 2 09:00–19:00 UTC Jan 3 10:00–16:00, 18:00–22:00 UTC Jan 4 11:00–20:00 UTC Jan 5 09:00–14:00, 16:00–21:00 UTC Jan 6 10:00–20:00 UTC Jan 7 10:00–11:00, 13:00–17:00 UTC Jan 8 08:00–20:00 UTC Jan 9 10:00–16:00, 20:00–24:00 UTC Jan 10 00:00–02:00, 07:00–10:00, 15:00–18:00, 21:00–24:00 UTC Jan 11 07:00–15:00, 20:00–24:00 UTC Jan 12 00:00–01:00, 09:00–16:00, 20:00–23:00 UTC Jan 13 09:00–21:00 UTC Jan 14 10:00–16:00, 18:00–20:00 UTC Jan 15 15:00–22:00 UTC Jan 16 10:00–18:00, 22:00–24:00 UTC Jan 17 00:00–02:00, 09:00–14:00, 17:00–22:00 UTC Jan 18 10:00–21:00 UTC Jan 19 12:00–17:00, 19:00–24:00 UTC Jan 20 10:00–22:00 UTC Jan 21 13:00–21:00 UTC Jan 22 11:00–24:00 UTC Jan 23 00:00–01:00, 11:00–19:00, 22:00–24:00 UTC Jan 24 11:00–18:00, 20:00–23:00 UTC Jan 25 07:00–09:00, 17:00–18:00, 22:00–24:00 UTC Jan 26 09:00–16:00, 22:00–24:00 UTC Jan 27 10:00–14:00, 18:00–22:00 UTC Jan 28 09:00–13:00, 16:00–22:00 UTC Jan 29 12:00–18:00 UTC Jan 30 03:00–08:00, 14:00–18:00 UTC Jan 31 04:00–08:00, 14:00–18:00 UTC
February
Feb 1 05:00–09:00, 13:00–16:00 UTC Feb 2 03:00–18:00 UTC Feb 3 09:00–10:00, 13:00–15:00, 19:00–23:00 UTC Feb 4 11:00–24:00 UTC Feb 5 00:00–05:00, 15:00–20:00 UTC Feb 6 11:00–14:00 UTC Feb 7 09:00–16:00 UTC Feb 8 08:00–20:00 UTC Feb 9 09:00–21:00 UTC Feb 10 07:00–18:00 UTC Feb 11 08:00–17:00 UTC Feb 12 05:00–09:00 UTC Feb 13 08:00–21:00 UTC Feb 14 09:00–19:00 UTC Feb 15 09:00–18:00 UTC Feb 16 08:00–16:00 UTC Feb 17 09:00–18:00 UTC Feb 18 08:00–14:00, 16:00–20:00 UTC Feb 19 08:00–15:00 UTC Feb 20 07:00–11:00, 17:00–18:00 UTC Feb 21 08:00–09:00, 11:00–14:00 UTC Feb 22 08:00–11:00, 13:00–22:00 UTC Feb 23 08:00–09:00, 14:00–18:00 UTC Feb 24 08:00–14:00 UTC Feb 25 07:00–13:00, 16:00–20:00 UTC Feb 26 07:00–13:00, 15:00–20:00 UTC Feb 27 08:00–15:00, 18:00–19:00 UTC Feb 28 09:00–20:00 UTC
March
Mar 1 08:00–14:00, 16:00–22:00 UTC Mar 2 09:00–18:00 UTC Mar 3 08:00–10:00 UTC Mar 4 08:00–10:00, 17:00–22:00 UTC Mar 5 09:00–12:00, 17:00–18:00, 20:00–21:00 UTC Mar 6 09:00–10:00, 12:00–19:00, 21:00–22:00 UTC Mar 7 08:00–14:00, 18:00–23:00 UTC Mar 8 09:00–10:00, 14:00–21:00 UTC Mar 9 09:00–13:00 UTC Mar 10 10:00–21:00 UTC Mar 11 08:00–22:00 UTC Mar 12 09:00–16:00, 18:00–23:00 UTC Mar 13 07:00–08:00, 12:00–17:00 UTC Mar 15 09:00–20:00 UTC Mar 16 08:00–11:00, 15:00–20:00 UTC Mar 17 15:00–21:00 UTC Mar 18 09:00–19:00 UTC Mar 19 08:00–12:00, 14:00–21:00 UTC Mar 20 07:00–20:00 UTC Mar 21 07:00–13:00 UTC Mar 22 07:00–19:00 UTC Mar 23 09:00–21:00 UTC Mar 24 07:00–13:00, 15:00–20:00 UTC Mar 25 07:00–09:00, 15:00–17:00 UTC Mar 26 07:00–12:00, 15:00–21:00 UTC Mar 27 07:00–20:00 UTC Mar 28 08:00–13:00 UTC Mar 29 07:00–13:00, 16:00–19:00 UTC Mar 30 08:00–11:00, 15:00–20:00 UTC Mar 31 08:00–14:00, 17:00–19:00 UTC
April
Apr 1 07:00–09:00, 11:00–15:00 UTC Apr 2 08:00–20:00 UTC Apr 3 08:00–17:00 UTC Apr 4 07:00–17:00 UTC Apr 5 08:00–15:00 UTC Apr 6 07:00–17:00 UTC Apr 7 07:00–14:00, 17:00–18:00 UTC Apr 8 08:00–14:00 UTC Apr 9 07:00–17:00, 22:00–24:00 UTC Apr 10 08:00–20:00 UTC Apr 11 07:00–18:00 UTC Apr 12 07:00–14:00, 19:00–21:00 UTC Apr 13 08:00–14:00 UTC Apr 14 07:00–12:00, 17:00–18:00 UTC Apr 15 06:00–15:00 UTC Apr 16 06:00–17:00 UTC Apr 17 05:00–16:00 UTC Apr 18 07:00–10:00 UTC Apr 19 07:00–12:00 UTC Apr 20 07:00–15:00 UTC Apr 21 06:00–13:00, 15:00–17:00 UTC Apr 22 07:00–12:00, 15:00–16:00 UTC Apr 23 07:00–15:00 UTC Apr 24 05:00–10:00 UTC Apr 25 05:00–17:00 UTC Apr 26 07:00–18:00 UTC Apr 27 05:00–11:00 UTC Apr 28 06:00–17:00 UTC Apr 29 06:00–18:00 UTC Apr 30 06:00–12:00 UTC
May
May 2 04:00–13:00, 15:00–16:00 UTC May 3 06:00–13:00, 18:00–19:00 UTC May 4 06:00–14:00 UTC May 5 03:00–12:00 UTC May 6 04:00–12:00 UTC May 7 06:00–14:00 UTC May 8 05:00–14:00 UTC May 9 05:00–11:00 UTC May 10 05:00–10:00 UTC May 11 04:00–13:00, 16:00–18:00 UTC May 12 04:00–14:00 UTC May 13 04:00–11:00 UTC May 14 04:00–13:00 UTC May 15 04:00–07:00, 09:00–14:00 UTC May 16 03:00–09:00 UTC May 19 09:00–14:00, 16:00–17:00 UTC May 20 04:00–12:00, 15:00–17:00 UTC May 21 05:00–13:00, 16:00–17:00 UTC May 22 05:00–16:00 UTC May 23 05:00–14:00 UTC May 24 04:00–13:00 UTC May 25 04:00–12:00, 14:00–15:00 UTC May 26 05:00–11:00 UTC May 27 05:00–14:00 UTC May 28 04:00–16:00 UTC May 31 04:00–15:00 UTC
June
Jun 1 04:00–14:00 UTC Jun 2 04:00–12:00 UTC Jun 3 04:00–08:00, 11:00–16:00 UTC Jun 4 04:00–13:00 UTC Jun 5 05:00–13:00 UTC Jun 6 04:00–14:00 UTC Jun 7 05:00–14:00 UTC Jun 8 14:00–17:00 UTC Jun 9 05:00–13:00 UTC Jun 10 04:00–16:00 UTC Jun 11 08:00–15:00 UTC Jun 12 05:00–10:00, 14:00–18:00 UTC Jun 13 06:00–12:00, 16:00–19:00 UTC Jun 15 07:00–12:00, 14:00–16:00 UTC Jun 16 02:00–13:00 UTC Jun 17 04:00–14:00 UTC Jun 18 05:00–12:00 UTC Jun 19 05:00–16:00 UTC Jun 20 05:00–14:00 UTC Jun 21 03:00–18:00 UTC Jun 22 04:00–15:00 UTC Jun 23 06:00–13:00 UTC Jun 24 04:00–12:00, 15:00–16:00 UTC Jun 25 05:00–11:00 UTC Jun 26 03:00–14:00 UTC Jun 27 04:00–12:00 UTC Jun 28 05:00–07:00 UTC Jun 29 04:00–09:00, 11:00–17:00 UTC Jun 30 05:00–12:00, 15:00–20:00 UTC
July
Jul 1 05:00–12:00 UTC Jul 2 05:00–15:00 UTC Jul 3 05:00–12:00 UTC Jul 4 05:00–07:00 UTC Jul 5 07:00–14:00 UTC Jul 6 05:00–14:00 UTC Jul 7 03:00–11:00 UTC Jul 8 05:00–12:00 UTC Jul 9 05:00–13:00 UTC Jul 10 04:00–11:00 UTC Jul 11 06:00–12:00 UTC Jul 13 05:00–07:00 UTC Jul 14 07:00–12:00 UTC Jul 15 08:00–09:00 UTC Jul 16 07:00–20:00 UTC Jul 17 04:00–06:00, 08:00–13:00 UTC Jul 18 06:00–10:00 UTC Jul 19 06:00–12:00 UTC Jul 20 07:00–10:00, 14:00–16:00, 18:00–22:00 UTC Jul 21 11:00–12:00 UTC Jul 22 05:00–10:00, 16:00–20:00 UTC Jul 23 08:00–10:00, 13:00–21:00 UTC Jul 25 11:00–13:00, 17:00–19:00 UTC Jul 26 07:00–10:00, 14:00–17:00 UTC Jul 27 06:00–09:00, 17:00–18:00 UTC