Most often in October, 29 days.
Highest follower count seen that year: 2,455.
August
Aug 19 14:00–18:00, 20:00–23:00 UTC Aug 20 00:00–03:00, 19:00–20:00, 23:00–24:00 UTC Aug 21 00:00–03:00, 19:00–23:00 UTC Aug 22 00:00–02:00 UTC Aug 24 21:00–24:00 UTC Aug 25 01:00–02:00, 21:00–23:00 UTC Aug 28 17:00–24:00 UTC Aug 29 15:00–16:00, 20:00–22:00 UTC Aug 30 16:00–24:00 UTC Aug 31 00:00–01:00, 12:00–19:00, 23:00–24:00 UTC
September
Sep 1 00:00–02:00, 16:00–17:00, 19:00–23:00 UTC Sep 2 02:00–08:00, 15:00–16:00 UTC Sep 3 17:00–19:00, 21:00–23:00 UTC Sep 4 02:00–04:00, 18:00–21:00 UTC Sep 5 01:00–03:00, 12:00–17:00, 19:00–24:00 UTC Sep 6 00:00–01:00, 22:00–24:00 UTC Sep 7 00:00–01:00 UTC Sep 8 22:00–24:00 UTC Sep 9 00:00–01:00, 21:00–23:00 UTC Sep 15 18:00–20:00 UTC Sep 16 08:00–11:00, 17:00–18:00, 20:00–22:00 UTC Sep 17 15:00–17:00 UTC Sep 18 10:00–12:00, 14:00–15:00, 18:00–22:00 UTC Sep 19 21:00–24:00 UTC Sep 20 22:00–24:00 UTC Sep 21 00:00–01:00, 09:00–11:00 UTC Sep 25 14:00–15:00 UTC Sep 26 23:00–24:00 UTC Sep 27 01:00–02:00, 15:00–17:00, 20:00–21:00 UTC Sep 28 16:00–17:00, 19:00–21:00 UTC Sep 29 15:00–17:00, 20:00–23:00 UTC Sep 30 13:00–14:00, 17:00–20:00 UTC
October
Oct 1 01:00–03:00, 19:00–23:00 UTC Oct 2 01:00–03:00, 21:00–24:00 UTC Oct 3 00:00–01:00, 17:00–20:00, 22:00–24:00 UTC Oct 4 20:00–21:00 UTC Oct 6 23:00–24:00 UTC Oct 7 00:00–03:00, 16:00–17:00, 22:00–24:00 UTC Oct 8 00:00–01:00, 21:00–24:00 UTC Oct 9 00:00–01:00, 20:00–21:00 UTC Oct 10 10:00–11:00, 16:00–18:00 UTC Oct 11 16:00–19:00, 21:00–22:00 UTC Oct 12 20:00–21:00 UTC Oct 13 18:00–22:00 UTC Oct 14 18:00–19:00 UTC Oct 15 13:00–19:00, 23:00–24:00 UTC Oct 16 00:00–01:00, 20:00–24:00 UTC Oct 17 19:00–20:00, 22:00–23:00 UTC Oct 18 01:00–02:00, 13:00–22:00 UTC Oct 19 01:00–02:00, 13:00–18:00 UTC Oct 20 00:00–02:00, 21:00–22:00 UTC Oct 21 20:00–21:00 UTC Oct 22 20:00–21:00 UTC Oct 23 15:00–21:00 UTC Oct 25 04:00–06:00 UTC Oct 26 17:00–22:00 UTC Oct 27 13:00–14:00, 21:00–22:00 UTC Oct 28 14:00–18:00, 21:00–23:00 UTC Oct 29 02:00–03:00, 14:00–16:00, 18:00–20:00 UTC Oct 30 15:00–18:00, 20:00–22:00 UTC Oct 31 03:00–06:00, 20:00–21:00 UTC
November
Nov 1 19:00–21:00 UTC Nov 2 02:00–04:00, 16:00–23:00 UTC Nov 3 02:00–03:00, 07:00–08:00, 16:00–22:00 UTC Nov 5 16:00–17:00, 21:00–24:00 UTC Nov 6 18:00–19:00, 21:00–22:00 UTC Nov 8 07:00–08:00, 15:00–16:00 UTC Nov 9 16:00–17:00, 19:00–22:00 UTC Nov 10 18:00–22:00 UTC Nov 11 16:00–18:00, 20:00–23:00 UTC Nov 12 18:00–23:00 UTC Nov 13 17:00–21:00 UTC Nov 14 19:00–23:00 UTC Nov 15 16:00–19:00, 21:00–22:00 UTC Nov 16 16:00–17:00 UTC Nov 17 17:00–22:00 UTC Nov 18 16:00–17:00, 23:00–24:00 UTC Nov 19 00:00–02:00 UTC Nov 21 15:00–21:00 UTC Nov 22 17:00–19:00, 21:00–22:00 UTC Nov 23 13:00–15:00, 19:00–21:00 UTC Nov 24 15:00–17:00, 19:00–21:00 UTC Nov 25 17:00–18:00 UTC Nov 26 02:00–03:00, 14:00–18:00 UTC Nov 27 15:00–19:00 UTC Nov 28 01:00–03:00, 23:00–24:00 UTC Nov 29 15:00–18:00 UTC Nov 30 18:00–22:00 UTC
December
Dec 1 12:00–14:00 UTC Dec 2 14:00–15:00, 20:00–22:00 UTC Dec 3 15:00–17:00, 20:00–22:00 UTC Dec 4 15:00–19:00 UTC Dec 5 17:00–19:00 UTC Dec 6 15:00–17:00, 20:00–21:00 UTC Dec 7 17:00–19:00 UTC Dec 8 12:00–14:00, 17:00–20:00 UTC Dec 9 16:00–20:00 UTC Dec 11 21:00–24:00 UTC Dec 12 15:00–16:00, 19:00–20:00 UTC Dec 13 18:00–20:00 UTC Dec 15 17:00–19:00 UTC Dec 16 20:00–21:00 UTC Dec 18 04:00–07:00, 20:00–22:00 UTC Dec 19 12:00–13:00, 17:00–19:00 UTC Dec 20 00:00–01:00, 19:00–20:00, 23:00–24:00 UTC Dec 21 00:00–01:00, 20:00–24:00 UTC Dec 22 19:00–22:00 UTC Dec 23 20:00–21:00 UTC Dec 24 12:00–15:00, 18:00–23:00 UTC