Most often in September, 29 days.
Highest follower count seen that year: 16,582.
July
Jul 28 19:00–24:00 UTC Jul 29 00:00–02:00, 19:00–24:00 UTC Jul 30 01:00–02:00, 19:00–24:00 UTC Jul 31 00:00–02:00, 19:00–24:00 UTC
August
Aug 1 00:00–01:00, 20:00–24:00 UTC Aug 2 00:00–02:00, 19:00–24:00 UTC Aug 3 00:00–02:00, 13:00–20:00 UTC Aug 4 19:00–24:00 UTC Aug 5 19:00–24:00 UTC Aug 6 00:00–02:00, 20:00–24:00 UTC Aug 7 00:00–01:00 UTC Aug 9 19:00–24:00 UTC Aug 10 00:00–02:00, 14:00–23:00 UTC Aug 12 19:00–24:00 UTC Aug 13 00:00–02:00, 20:00–24:00 UTC Aug 14 00:00–02:00, 20:00–24:00 UTC Aug 15 00:00–02:00, 19:00–24:00 UTC Aug 16 00:00–02:00, 19:00–24:00 UTC Aug 17 00:00–02:00, 13:00–19:00 UTC Aug 20 13:00–20:00 UTC Aug 21 19:00–24:00 UTC Aug 22 00:00–02:00, 21:00–24:00 UTC Aug 23 00:00–03:00, 20:00–24:00 UTC Aug 24 00:00–01:00, 13:00–18:00 UTC Aug 25 19:00–23:00 UTC Aug 26 19:00–24:00 UTC Aug 27 00:00–02:00, 19:00–24:00 UTC Aug 28 00:00–02:00, 19:00–24:00 UTC Aug 29 00:00–02:00, 20:00–24:00 UTC Aug 30 00:00–01:00, 18:00–24:00 UTC Aug 31 00:00–01:00, 13:00–20:00 UTC
September
Sep 1 19:00–24:00 UTC Sep 2 00:00–02:00, 19:00–24:00 UTC Sep 3 00:00–01:00, 20:00–24:00 UTC Sep 4 00:00–02:00, 20:00–24:00 UTC Sep 5 00:00–02:00, 19:00–24:00 UTC Sep 6 00:00–02:00, 20:00–24:00 UTC Sep 7 00:00–02:00, 13:00–20:00 UTC Sep 8 19:00–24:00 UTC Sep 9 00:00–01:00, 19:00–24:00 UTC Sep 10 00:00–02:00, 20:00–24:00 UTC Sep 11 00:00–02:00, 20:00–24:00 UTC Sep 12 00:00–01:00 UTC Sep 14 01:00–10:00 UTC Sep 15 20:00–24:00 UTC Sep 16 00:00–01:00, 20:00–24:00 UTC Sep 17 00:00–03:00, 20:00–24:00 UTC Sep 18 00:00–02:00 UTC Sep 19 20:00–24:00 UTC Sep 20 00:00–03:00, 20:00–24:00 UTC Sep 21 00:00–02:00, 14:00–20:00 UTC Sep 22 20:00–24:00 UTC Sep 23 00:00–02:00 UTC Sep 24 19:00–24:00 UTC Sep 25 00:00–01:00, 19:00–24:00 UTC Sep 26 00:00–02:00, 19:00–24:00 UTC Sep 27 00:00–02:00, 20:00–24:00 UTC Sep 28 00:00–03:00, 13:00–18:00 UTC Sep 29 20:00–24:00 UTC Sep 30 00:00–03:00, 19:00–24:00 UTC
October
Oct 1 00:00–01:00, 19:00–24:00 UTC Oct 2 00:00–02:00, 20:00–24:00 UTC Oct 3 00:00–02:00, 19:00–24:00 UTC Oct 4 00:00–02:00, 19:00–24:00 UTC Oct 5 14:00–19:00 UTC Oct 6 20:00–24:00 UTC Oct 7 00:00–02:00, 19:00–24:00 UTC Oct 8 00:00–02:00, 20:00–24:00 UTC Oct 9 00:00–01:00, 20:00–24:00 UTC Oct 10 00:00–02:00, 20:00–24:00 UTC Oct 11 00:00–02:00, 20:00–24:00 UTC Oct 13 20:00–24:00 UTC Oct 14 00:00–02:00, 20:00–24:00 UTC Oct 15 00:00–02:00, 20:00–24:00 UTC Oct 16 00:00–01:00, 20:00–24:00 UTC Oct 17 00:00–02:00, 20:00–24:00 UTC Oct 18 00:00–01:00, 20:00–24:00 UTC Oct 19 00:00–02:00, 15:00–22:00 UTC Oct 21 21:00–24:00 UTC Oct 22 13:00–19:00 UTC Oct 23 11:00–20:00 UTC Oct 24 11:00–18:00 UTC Oct 25 11:00–21:00 UTC Oct 27 11:00–20:00 UTC Oct 28 11:00–17:00 UTC Oct 30 11:00–21:00 UTC Oct 31 11:00–21:00 UTC
November
Nov 1 11:00–21:00 UTC Nov 3 12:00–19:00 UTC Nov 4 11:00–21:00 UTC Nov 5 11:00–17:00 UTC Nov 6 11:00–21:00 UTC Nov 7 11:00–19:00 UTC Nov 8 11:00–21:00 UTC Nov 10 11:00–20:00 UTC Nov 11 10:00–21:00 UTC Nov 12 11:00–21:00 UTC Nov 13 11:00–21:00 UTC Nov 14 12:00–22:00 UTC Nov 16 14:00–19:00 UTC Nov 17 13:00–19:00 UTC Nov 18 11:00–19:00 UTC Nov 19 11:00–18:00 UTC Nov 20 11:00–21:00 UTC Nov 21 11:00–20:00 UTC Nov 22 11:00–19:00 UTC Nov 24 11:00–18:00 UTC Nov 25 11:00–20:00 UTC Nov 26 11:00–19:00 UTC Nov 28 11:00–13:00 UTC
December
Dec 1 11:00–21:00 UTC Dec 2 11:00–21:00 UTC Dec 3 12:00–20:00 UTC Dec 4 11:00–18:00 UTC Dec 5 14:00–20:00 UTC Dec 6 11:00–21:00 UTC Dec 8 13:00–19:00 UTC Dec 9 11:00–16:00 UTC Dec 10 11:00–18:00 UTC Dec 11 11:00–21:00 UTC Dec 12 14:00–21:00 UTC Dec 13 12:00–20:00 UTC Dec 14 13:00–20:00 UTC Dec 15 11:00–24:00 UTC Dec 17 11:00–17:00 UTC Dec 18 11:00–20:00 UTC Dec 19 12:00–24:00 UTC Dec 20 11:00–23:00 UTC Dec 21 12:00–21:00 UTC Dec 22 11:00–18:00 UTC Dec 23 11:00–19:00 UTC Dec 24 12:00–22:00 UTC Dec 26 12:00–20:00 UTC Dec 27 13:00–17:00 UTC Dec 28 05:00–10:00, 13:00–19:00 UTC Dec 29 13:00–20:00 UTC Dec 30 11:00–22:00 UTC