Most often in October, 29 days.
Highest follower count seen that year: 31,163.
July
Jul 28 07:00–08:00, 13:00–18:00 UTC Jul 29 21:00–24:00 UTC Jul 30 01:00–02:00, 15:00–19:00, 22:00–23:00 UTC
August
Aug 2 21:00–24:00 UTC Aug 3 00:00–01:00 UTC Aug 4 15:00–19:00, 23:00–24:00 UTC Aug 5 11:00–18:00, 22:00–23:00 UTC Aug 6 04:00–08:00, 15:00–17:00 UTC Aug 7 16:00–21:00 UTC Aug 8 04:00–05:00 UTC Aug 9 07:00–14:00, 17:00–20:00 UTC Aug 11 16:00–19:00, 23:00–24:00 UTC Aug 12 00:00–01:00 UTC Aug 13 13:00–14:00, 17:00–19:00 UTC Aug 14 16:00–20:00 UTC Aug 15 03:00–04:00, 14:00–16:00 UTC Aug 16 04:00–05:00, 12:00–15:00, 18:00–20:00, 22:00–23:00 UTC Aug 17 04:00–06:00, 18:00–19:00, 22:00–24:00 UTC Aug 18 00:00–02:00, 17:00–19:00, 21:00–24:00 UTC Aug 19 17:00–19:00 UTC Aug 20 00:00–03:00, 23:00–24:00 UTC Aug 21 00:00–01:00, 14:00–17:00, 21:00–24:00 UTC Aug 22 05:00–06:00, 14:00–18:00 UTC Aug 23 15:00–20:00, 23:00–24:00 UTC Aug 24 01:00–02:00, 07:00–09:00, 13:00–18:00, 20:00–21:00 UTC Aug 25 02:00–03:00, 17:00–20:00 UTC Aug 28 17:00–19:00, 21:00–22:00 UTC Aug 29 01:00–03:00, 09:00–10:00, 18:00–19:00 UTC Aug 30 02:00–05:00, 15:00–18:00 UTC
September
Sep 2 16:00–22:00 UTC Sep 3 16:00–17:00 UTC Sep 8 23:00–24:00 UTC Sep 9 00:00–03:00 UTC Sep 10 07:00–12:00 UTC Sep 12 08:00–16:00 UTC Sep 13 09:00–13:00 UTC Sep 14 17:00–21:00 UTC Sep 15 01:00–05:00 UTC Sep 16 05:00–08:00, 16:00–20:00 UTC Sep 17 03:00–04:00, 11:00–13:00, 15:00–17:00 UTC Sep 18 17:00–22:00 UTC Sep 19 02:00–03:00, 09:00–15:00, 19:00–20:00 UTC Sep 20 15:00–16:00 UTC Sep 21 00:00–03:00, 05:00–06:00, 17:00–19:00, 22:00–24:00 UTC Sep 22 11:00–13:00, 15:00–17:00 UTC Sep 23 15:00–18:00, 21:00–24:00 UTC Sep 24 14:00–18:00 UTC Sep 25 09:00–10:00, 16:00–19:00 UTC Sep 26 19:00–21:00 UTC Sep 27 01:00–04:00, 12:00–13:00, 18:00–21:00 UTC Sep 28 03:00–04:00 UTC Sep 29 08:00–11:00, 16:00–24:00 UTC Sep 30 00:00–04:00, 16:00–21:00 UTC
October
Oct 1 03:00–11:00, 13:00–17:00 UTC Oct 2 17:00–19:00 UTC Oct 3 07:00–10:00, 15:00–18:00 UTC Oct 4 17:00–24:00 UTC Oct 5 00:00–01:00, 06:00–08:00, 15:00–17:00 UTC Oct 6 00:00–07:00, 16:00–23:00 UTC Oct 7 19:00–24:00 UTC Oct 8 11:00–15:00, 18:00–19:00 UTC Oct 9 01:00–02:00 UTC Oct 10 11:00–13:00, 15:00–19:00 UTC Oct 12 15:00–19:00 UTC Oct 13 17:00–20:00, 22:00–24:00 UTC Oct 14 10:00–12:00 UTC Oct 15 00:00–02:00, 21:00–24:00 UTC Oct 16 00:00–04:00, 17:00–22:00 UTC Oct 17 00:00–05:00, 07:00–08:00, 17:00–19:00 UTC Oct 18 01:00–02:00, 08:00–09:00, 21:00–23:00 UTC Oct 20 19:00–24:00 UTC Oct 21 15:00–17:00 UTC Oct 22 05:00–06:00, 15:00–18:00 UTC Oct 23 14:00–16:00, 20:00–22:00 UTC Oct 24 03:00–05:00, 13:00–24:00 UTC Oct 25 00:00–01:00, 04:00–06:00, 19:00–23:00 UTC Oct 26 04:00–06:00, 16:00–20:00 UTC Oct 27 07:00–09:00, 17:00–24:00 UTC Oct 28 16:00–19:00 UTC Oct 29 03:00–06:00, 17:00–22:00 UTC Oct 30 02:00–06:00 UTC Oct 31 11:00–14:00, 18:00–21:00 UTC
November
Nov 1 03:00–07:00 UTC Nov 2 01:00–03:00, 06:00–08:00 UTC Nov 3 17:00–22:00 UTC Nov 4 04:00–08:00, 18:00–21:00 UTC Nov 5 13:00–16:00 UTC Nov 6 04:00–11:00, 15:00–19:00 UTC Nov 7 18:00–21:00 UTC Nov 8 04:00–07:00 UTC Nov 9 16:00–18:00 UTC Nov 13 16:00–18:00 UTC Nov 14 17:00–20:00 UTC Nov 15 03:00–04:00 UTC Nov 16 02:00–03:00, 21:00–23:00 UTC Nov 17 18:00–19:00 UTC Nov 20 07:00–09:00, 23:00–24:00 UTC Nov 21 00:00–04:00 UTC Nov 23 01:00–04:00 UTC Nov 25 23:00–24:00 UTC Nov 26 00:00–02:00, 23:00–24:00 UTC Nov 27 00:00–03:00, 15:00–17:00, 21:00–24:00 UTC Nov 28 00:00–03:00, 10:00–17:00, 22:00–24:00 UTC Nov 29 01:00–05:00, 19:00–24:00 UTC Nov 30 11:00–13:00, 16:00–24:00 UTC
December
Dec 1 00:00–01:00, 16:00–21:00 UTC Dec 2 03:00–04:00, 18:00–20:00 UTC Dec 3 07:00–08:00, 16:00–19:00 UTC Dec 4 16:00–19:00 UTC Dec 5 00:00–05:00, 18:00–23:00 UTC Dec 6 00:00–04:00, 12:00–14:00, 16:00–17:00, 21:00–24:00 UTC Dec 7 00:00–02:00, 18:00–24:00 UTC Dec 8 00:00–01:00, 19:00–22:00 UTC Dec 9 10:00–12:00, 17:00–22:00 UTC Dec 10 02:00–05:00, 17:00–21:00 UTC Dec 11 02:00–03:00, 10:00–13:00, 19:00–24:00 UTC Dec 12 17:00–19:00 UTC Dec 13 09:00–13:00, 20:00–23:00 UTC Dec 14 00:00–01:00, 12:00–13:00, 18:00–20:00 UTC Dec 16 14:00–22:00 UTC Dec 17 18:00–21:00 UTC Dec 18 03:00–04:00, 21:00–24:00 UTC Dec 19 00:00–01:00, 09:00–11:00, 16:00–19:00 UTC Dec 21 00:00–03:00, 09:00–12:00, 15:00–16:00 UTC Dec 22 23:00–24:00 UTC Dec 23 00:00–01:00 UTC Dec 24 17:00–19:00 UTC Dec 26 12:00–14:00, 16:00–24:00 UTC Dec 27 16:00–21:00 UTC Dec 29 14:00–18:00 UTC Dec 30 11:00–13:00, 16:00–17:00 UTC Dec 31 16:00–18:00 UTC