Most often in August, 30 days.
Highest follower count seen that year: 8,923.
July
Jul 28 00:00–02:00, 22:00–24:00 UTC Jul 29 00:00–01:00, 23:00–24:00 UTC Jul 30 01:00–02:00, 21:00–23:00 UTC
August
Aug 2 00:00–02:00, 23:00–24:00 UTC Aug 3 00:00–03:00, 22:00–24:00 UTC Aug 4 00:00–01:00, 22:00–24:00 UTC Aug 5 20:00–24:00 UTC Aug 6 21:00–24:00 UTC Aug 7 00:00–01:00, 16:00–18:00, 22:00–24:00 UTC Aug 8 00:00–02:00, 22:00–24:00 UTC Aug 9 00:00–01:00, 21:00–24:00 UTC Aug 10 00:00–01:00, 22:00–24:00 UTC Aug 11 04:00–05:00, 20:00–21:00 UTC Aug 12 21:00–24:00 UTC Aug 13 00:00–01:00, 20:00–24:00 UTC Aug 14 00:00–01:00, 22:00–24:00 UTC Aug 15 00:00–01:00, 22:00–24:00 UTC Aug 16 00:00–02:00 UTC Aug 17 00:00–02:00, 17:00–22:00 UTC Aug 18 00:00–01:00, 20:00–24:00 UTC Aug 19 00:00–01:00, 21:00–24:00 UTC Aug 20 00:00–01:00, 23:00–24:00 UTC Aug 21 00:00–02:00 UTC Aug 22 00:00–01:00 UTC Aug 23 21:00–24:00 UTC Aug 24 00:00–02:00, 20:00–23:00 UTC Aug 25 22:00–24:00 UTC Aug 26 00:00–02:00, 21:00–24:00 UTC Aug 27 00:00–01:00, 15:00–17:00, 19:00–24:00 UTC Aug 28 00:00–01:00, 23:00–24:00 UTC Aug 29 00:00–02:00 UTC Aug 30 21:00–24:00 UTC Aug 31 00:00–02:00, 21:00–24:00 UTC
September
Sep 1 00:00–02:00 UTC Sep 2 21:00–24:00 UTC Sep 3 00:00–01:00, 21:00–24:00 UTC Sep 4 00:00–01:00 UTC Sep 5 23:00–24:00 UTC Sep 6 00:00–02:00, 23:00–24:00 UTC Sep 7 00:00–02:00, 22:00–24:00 UTC Sep 8 00:00–01:00, 23:00–24:00 UTC Sep 9 00:00–01:00, 22:00–24:00 UTC Sep 10 00:00–02:00, 22:00–24:00 UTC Sep 11 00:00–01:00 UTC Sep 12 22:00–24:00 UTC Sep 13 00:00–02:00, 23:00–24:00 UTC Sep 14 00:00–02:00, 21:00–24:00 UTC Sep 15 00:00–01:00, 22:00–24:00 UTC Sep 16 00:00–01:00, 23:00–24:00 UTC Sep 17 00:00–01:00, 21:00–24:00 UTC Sep 18 20:00–24:00 UTC Sep 19 00:00–01:00, 22:00–23:00 UTC Sep 20 22:00–24:00 UTC Sep 21 00:00–01:00, 19:00–24:00 UTC Sep 22 00:00–01:00 UTC Sep 23 01:00–05:00, 22:00–24:00 UTC Sep 24 00:00–02:00, 22:00–24:00 UTC Sep 25 00:00–02:00, 13:00–15:00, 22:00–24:00 UTC Sep 26 00:00–01:00, 21:00–24:00 UTC Sep 27 00:00–02:00, 20:00–24:00 UTC Sep 28 00:00–02:00, 15:00–20:00, 23:00–24:00 UTC Sep 29 00:00–02:00, 21:00–24:00 UTC Sep 30 00:00–01:00, 12:00–16:00, 21:00–23:00 UTC
October
Oct 2 00:00–02:00, 22:00–24:00 UTC Oct 3 00:00–02:00, 23:00–24:00 UTC Oct 4 00:00–03:00, 16:00–19:00 UTC Oct 5 01:00–03:00, 15:00–16:00, 21:00–24:00 UTC Oct 6 15:00–18:00, 22:00–24:00 UTC Oct 7 00:00–02:00, 19:00–24:00 UTC Oct 8 00:00–01:00, 13:00–18:00 UTC Oct 9 20:00–24:00 UTC Oct 10 00:00–02:00, 21:00–24:00 UTC Oct 11 00:00–02:00, 21:00–24:00 UTC Oct 12 00:00–02:00, 13:00–15:00, 19:00–24:00 UTC Oct 13 00:00–03:00, 18:00–23:00 UTC Oct 14 20:00–24:00 UTC Oct 15 00:00–02:00, 20:00–24:00 UTC Oct 16 00:00–02:00, 20:00–24:00 UTC Oct 17 00:00–01:00 UTC Oct 18 00:00–03:00 UTC Oct 19 00:00–02:00, 14:00–17:00, 19:00–24:00 UTC Oct 20 00:00–01:00, 20:00–24:00 UTC Oct 21 00:00–02:00, 22:00–24:00 UTC Oct 22 00:00–02:00, 13:00–18:00, 23:00–24:00 UTC Oct 23 01:00–02:00, 22:00–24:00 UTC Oct 24 00:00–01:00 UTC Oct 25 00:00–01:00, 21:00–22:00 UTC Oct 26 20:00–24:00 UTC Oct 27 00:00–01:00, 21:00–24:00 UTC Oct 28 00:00–02:00 UTC Oct 29 00:00–02:00, 20:00–24:00 UTC Oct 30 00:00–01:00 UTC
November
Nov 1 21:00–24:00 UTC Nov 2 00:00–03:00, 14:00–18:00, 22:00–24:00 UTC Nov 3 00:00–02:00 UTC Nov 4 20:00–23:00 UTC Nov 5 19:00–24:00 UTC Nov 6 16:00–18:00, 22:00–24:00 UTC Nov 7 00:00–01:00, 22:00–24:00 UTC Nov 8 00:00–02:00, 15:00–18:00, 22:00–24:00 UTC Nov 9 00:00–02:00, 15:00–19:00 UTC Nov 10 23:00–24:00 UTC Nov 11 00:00–02:00, 22:00–24:00 UTC Nov 12 00:00–02:00 UTC Nov 13 13:00–17:00, 23:00–24:00 UTC Nov 14 00:00–02:00, 13:00–15:00, 21:00–24:00 UTC Nov 15 00:00–01:00, 20:00–24:00 UTC Nov 16 00:00–02:00, 17:00–19:00, 22:00–24:00 UTC Nov 17 00:00–02:00, 18:00–24:00 UTC Nov 18 00:00–02:00, 23:00–24:00 UTC Nov 19 00:00–02:00, 22:00–24:00 UTC Nov 20 21:00–24:00 UTC Nov 21 00:00–01:00, 23:00–24:00 UTC Nov 22 00:00–03:00, 22:00–24:00 UTC Nov 23 00:00–02:00, 22:00–24:00 UTC Nov 24 00:00–02:00 UTC Nov 25 21:00–24:00 UTC Nov 27 21:00–24:00 UTC Nov 28 00:00–02:00, 22:00–24:00 UTC Nov 29 00:00–02:00, 21:00–23:00 UTC Nov 30 00:00–02:00, 22:00–24:00 UTC
December
Dec 1 00:00–02:00, 22:00–24:00 UTC Dec 2 00:00–01:00, 21:00–23:00 UTC Dec 3 23:00–24:00 UTC Dec 4 00:00–02:00 UTC Dec 5 23:00–24:00 UTC Dec 6 00:00–02:00, 14:00–18:00 UTC Dec 7 01:00–02:00, 20:00–24:00 UTC Dec 8 00:00–02:00, 19:00–24:00 UTC Dec 9 00:00–02:00 UTC Dec 11 22:00–24:00 UTC Dec 12 00:00–02:00, 15:00–17:00 UTC Dec 13 12:00–18:00 UTC Dec 14 23:00–24:00 UTC Dec 15 00:00–02:00 UTC Dec 16 00:00–02:00, 16:00–18:00, 22:00–24:00 UTC Dec 17 00:00–03:00, 14:00–16:00 UTC Dec 18 01:00–03:00, 15:00–19:00 UTC Dec 20 20:00–22:00 UTC Dec 23 11:00–15:00 UTC Dec 31 14:00–16:00 UTC