Most often in September, 28 days.
Highest follower count seen that year: 7,450.
July
Jul 29 00:00–02:00, 23:00–24:00 UTC Jul 30 00:00–01:00 UTC Jul 31 01:00–02:00 UTC
August
Aug 2 00:00–02:00 UTC Aug 3 00:00–03:00, 14:00–16:00 UTC Aug 4 17:00–18:00, 23:00–24:00 UTC Aug 6 22:00–24:00 UTC Aug 7 00:00–01:00 UTC Aug 8 16:00–18:00 UTC Aug 9 17:00–19:00, 23:00–24:00 UTC Aug 10 00:00–01:00, 16:00–18:00 UTC Aug 11 22:00–24:00 UTC Aug 13 00:00–02:00, 17:00–19:00 UTC Aug 16 23:00–24:00 UTC Aug 17 00:00–02:00 UTC Aug 19 16:00–18:00 UTC Aug 20 00:00–01:00 UTC Aug 22 22:00–24:00 UTC Aug 23 00:00–01:00, 23:00–24:00 UTC Aug 24 00:00–01:00, 17:00–18:00 UTC Aug 26 23:00–24:00 UTC Aug 27 00:00–01:00, 17:00–18:00, 23:00–24:00 UTC Aug 28 00:00–02:00 UTC Aug 29 00:00–02:00, 15:00–17:00 UTC Aug 30 16:00–17:00, 23:00–24:00 UTC Aug 31 00:00–01:00, 16:00–18:00 UTC
September
Sep 1 17:00–19:00 UTC Sep 2 17:00–18:00, 23:00–24:00 UTC Sep 3 00:00–01:00, 17:00–18:00, 23:00–24:00 UTC Sep 4 00:00–01:00, 22:00–24:00 UTC Sep 5 00:00–02:00, 17:00–18:00, 22:00–24:00 UTC Sep 6 00:00–02:00, 18:00–19:00 UTC Sep 7 00:00–02:00, 17:00–18:00 UTC Sep 8 17:00–19:00 UTC Sep 9 00:00–02:00, 22:00–24:00 UTC Sep 10 00:00–02:00, 22:00–24:00 UTC Sep 11 21:00–24:00 UTC Sep 12 00:00–01:00, 17:00–18:00 UTC Sep 13 01:00–02:00, 15:00–18:00, 23:00–24:00 UTC Sep 14 00:00–01:00, 16:00–17:00 UTC Sep 16 00:00–01:00 UTC Sep 17 17:00–18:00 UTC Sep 18 00:00–01:00, 17:00–18:00 UTC Sep 19 23:00–24:00 UTC Sep 20 16:00–18:00 UTC Sep 21 00:00–01:00 UTC Sep 22 16:00–18:00 UTC Sep 23 00:00–02:00, 23:00–24:00 UTC Sep 24 00:00–01:00, 23:00–24:00 UTC Sep 25 00:00–01:00, 23:00–24:00 UTC Sep 26 00:00–01:00, 15:00–16:00 UTC Sep 27 00:00–02:00, 17:00–18:00, 23:00–24:00 UTC Sep 28 00:00–01:00, 16:00–18:00, 22:00–24:00 UTC Sep 30 17:00–19:00 UTC
October
Oct 1 17:00–19:00, 23:00–24:00 UTC Oct 2 00:00–01:00, 22:00–24:00 UTC Oct 3 00:00–01:00, 19:00–20:00 UTC Oct 4 00:00–02:00, 17:00–18:00 UTC Oct 5 00:00–03:00, 16:00–18:00, 22:00–23:00 UTC Oct 7 00:00–01:00 UTC Oct 8 00:00–01:00, 17:00–18:00 UTC Oct 9 01:00–02:00, 23:00–24:00 UTC Oct 10 00:00–02:00, 17:00–19:00, 23:00–24:00 UTC Oct 11 00:00–01:00, 17:00–19:00, 23:00–24:00 UTC Oct 12 00:00–01:00, 19:00–20:00 UTC Oct 14 00:00–02:00, 16:00–18:00, 22:00–24:00 UTC Oct 15 00:00–02:00 UTC Oct 16 17:00–18:00, 20:00–24:00 UTC Oct 17 00:00–01:00 UTC Oct 18 00:00–02:00 UTC Oct 19 00:00–02:00 UTC Oct 20 23:00–24:00 UTC Oct 21 16:00–18:00 UTC Oct 23 01:00–03:00, 22:00–24:00 UTC Oct 24 00:00–01:00 UTC Oct 25 00:00–02:00 UTC Oct 26 01:00–02:00 UTC Oct 27 00:00–04:00, 23:00–24:00 UTC Oct 28 00:00–02:00 UTC
November
Nov 11 00:00–02:00 UTC Nov 12 00:00–02:00 UTC Nov 13 00:00–01:00 UTC Nov 14 00:00–02:00, 16:00–17:00 UTC Nov 15 00:00–02:00, 17:00–18:00, 22:00–24:00 UTC Nov 16 00:00–01:00 UTC Nov 17 17:00–18:00, 22:00–24:00 UTC Nov 18 00:00–01:00, 22:00–24:00 UTC Nov 19 00:00–02:00, 23:00–24:00 UTC Nov 20 00:00–01:00 UTC Nov 21 00:00–02:00, 16:00–18:00 UTC Nov 22 01:00–02:00, 16:00–18:00 UTC Nov 23 00:00–02:00, 17:00–19:00, 23:00–24:00 UTC Nov 25 01:00–02:00 UTC Nov 26 00:00–02:00, 23:00–24:00 UTC Nov 27 00:00–02:00, 16:00–18:00 UTC Nov 28 01:00–02:00, 17:00–19:00 UTC Nov 29 01:00–02:00, 17:00–18:00 UTC Nov 30 01:00–02:00 UTC
December
Dec 1 17:00–19:00 UTC Dec 2 01:00–02:00 UTC Dec 3 00:00–02:00, 17:00–19:00 UTC Dec 4 00:00–02:00, 23:00–24:00 UTC Dec 5 00:00–02:00 UTC Dec 6 01:00–02:00, 17:00–18:00 UTC Dec 7 00:00–03:00, 21:00–22:00 UTC Dec 8 23:00–24:00 UTC Dec 9 00:00–01:00, 16:00–17:00 UTC Dec 10 00:00–02:00, 23:00–24:00 UTC Dec 11 00:00–02:00 UTC Dec 12 01:00–03:00, 17:00–18:00 UTC Dec 13 22:00–24:00 UTC Dec 14 00:00–01:00, 17:00–19:00 UTC Dec 16 00:00–02:00, 16:00–17:00 UTC Dec 17 00:00–02:00 UTC Dec 18 00:00–02:00, 17:00–18:00 UTC Dec 19 00:00–02:00, 17:00–18:00 UTC Dec 20 15:00–18:00 UTC Dec 21 00:00–02:00, 22:00–24:00 UTC Dec 23 00:00–02:00 UTC Dec 24 00:00–02:00, 22:00–24:00 UTC Dec 26 15:00–17:00 UTC Dec 28 00:00–02:00, 17:00–19:00 UTC Dec 31 01:00–02:00 UTC