Most often in August, 31 days.
Highest follower count seen that year: 27,243.
July
Jul 28 01:00–14:00 UTC Jul 29 01:00–13:00 UTC Jul 30 01:00–11:00 UTC Jul 31 01:00–11:00 UTC
August
Aug 1 01:00–11:00 UTC Aug 2 01:00–13:00 UTC Aug 3 01:00–12:00 UTC Aug 4 01:00–10:00 UTC Aug 5 06:00–12:00 UTC Aug 6 01:00–13:00 UTC Aug 7 01:00–14:00 UTC Aug 8 01:00–10:00 UTC Aug 9 01:00–13:00 UTC Aug 10 01:00–12:00 UTC Aug 11 01:00–10:00 UTC Aug 12 01:00–11:00 UTC Aug 13 01:00–12:00 UTC Aug 14 01:00–11:00, 13:00–16:00 UTC Aug 15 01:00–13:00 UTC Aug 16 01:00–12:00 UTC Aug 17 01:00–13:00 UTC Aug 18 01:00–13:00 UTC Aug 19 01:00–10:00 UTC Aug 20 02:00–12:00 UTC Aug 21 01:00–11:00 UTC Aug 22 01:00–13:00 UTC Aug 23 01:00–13:00 UTC Aug 24 01:00–12:00, 15:00–17:00 UTC Aug 25 01:00–13:00, 15:00–18:00 UTC Aug 26 01:00–07:00, 10:00–13:00, 16:00–18:00 UTC Aug 27 01:00–12:00, 15:00–18:00 UTC Aug 28 01:00–12:00, 15:00–18:00 UTC Aug 29 01:00–12:00, 14:00–16:00 UTC Aug 30 01:00–17:00 UTC Aug 31 01:00–12:00, 15:00–18:00 UTC
September
Sep 1 01:00–13:00, 15:00–18:00 UTC Sep 2 01:00–10:00 UTC Sep 3 01:00–11:00 UTC Sep 4 01:00–12:00 UTC Sep 5 01:00–07:00 UTC Sep 6 01:00–13:00 UTC Sep 7 02:00–10:00 UTC Sep 8 01:00–08:00 UTC Sep 9 01:00–11:00 UTC Sep 10 01:00–12:00, 15:00–18:00 UTC Sep 11 01:00–10:00, 15:00–17:00 UTC Sep 12 01:00–11:00 UTC Sep 13 01:00–11:00, 15:00–17:00 UTC Sep 14 01:00–12:00, 16:00–17:00 UTC Sep 15 01:00–13:00 UTC Sep 16 01:00–13:00 UTC Sep 17 01:00–07:00, 13:00–17:00 UTC Sep 18 01:00–14:00, 16:00–18:00 UTC Sep 19 01:00–07:00, 10:00–15:00 UTC Sep 20 01:00–13:00 UTC Sep 21 01:00–12:00, 15:00–17:00 UTC Sep 22 01:00–11:00 UTC Sep 23 01:00–11:00 UTC Sep 24 01:00–10:00 UTC Sep 25 01:00–07:00, 10:00–13:00 UTC Sep 26 02:00–13:00 UTC Sep 27 01:00–12:00, 15:00–17:00 UTC Sep 28 01:00–10:00 UTC Sep 29 01:00–12:00 UTC Sep 30 01:00–12:00, 15:00–17:00 UTC
October
Oct 1 01:00–11:00 UTC Oct 2 01:00–11:00 UTC Oct 3 01:00–14:00 UTC Oct 4 01:00–07:00, 10:00–13:00 UTC Oct 5 02:00–14:00 UTC Oct 6 01:00–15:00 UTC Oct 7 01:00–13:00 UTC Oct 8 01:00–08:00, 10:00–12:00 UTC Oct 9 01:00–13:00 UTC Oct 10 01:00–13:00 UTC Oct 11 02:00–12:00, 14:00–17:00 UTC Oct 12 01:00–13:00, 15:00–17:00 UTC Oct 13 01:00–14:00 UTC Oct 14 01:00–10:00, 15:00–17:00 UTC Oct 15 01:00–18:00 UTC Oct 16 01:00–10:00, 15:00–17:00 UTC Oct 17 01:00–11:00 UTC Oct 18 01:00–14:00 UTC Oct 19 01:00–09:00, 13:00–17:00 UTC Oct 20 01:00–12:00 UTC Oct 21 01:00–05:00, 08:00–11:00 UTC Oct 22 01:00–12:00 UTC Oct 23 01:00–14:00 UTC Oct 24 01:00–16:00 UTC Oct 25 01:00–07:00 UTC Oct 26 01:00–11:00, 14:00–16:00 UTC Oct 27 01:00–12:00, 14:00–17:00 UTC Oct 28 01:00–10:00, 15:00–18:00 UTC Oct 29 01:00–13:00 UTC
November
Nov 1 01:00–12:00 UTC Nov 2 01:00–14:00 UTC Nov 3 02:00–12:00 UTC Nov 4 01:00–11:00 UTC Nov 5 01:00–12:00 UTC Nov 6 01:00–12:00 UTC Nov 7 02:00–12:00 UTC Nov 8 01:00–12:00 UTC Nov 9 01:00–12:00, 15:00–17:00 UTC Nov 10 01:00–12:00, 22:00–24:00 UTC Nov 11 00:00–07:00 UTC Nov 14 01:00–11:00 UTC Nov 15 01:00–14:00 UTC Nov 16 01:00–11:00, 13:00–18:00 UTC Nov 17 01:00–14:00 UTC Nov 18 01:00–12:00 UTC Nov 19 01:00–13:00 UTC Nov 20 02:00–11:00 UTC Nov 21 01:00–13:00 UTC Nov 22 01:00–11:00 UTC Nov 23 01:00–10:00, 14:00–17:00 UTC Nov 24 01:00–12:00, 14:00–17:00 UTC Nov 25 01:00–11:00 UTC Nov 26 01:00–11:00 UTC Nov 27 01:00–14:00 UTC Nov 28 01:00–11:00 UTC Nov 29 01:00–13:00 UTC Nov 30 01:00–15:00 UTC
December
Dec 1 01:00–11:00 UTC Dec 2 01:00–10:00 UTC Dec 3 01:00–12:00 UTC Dec 4 01:00–12:00 UTC Dec 5 01:00–11:00 UTC Dec 6 01:00–12:00 UTC Dec 7 02:00–12:00 UTC Dec 8 01:00–11:00 UTC Dec 9 01:00–13:00 UTC Dec 10 01:00–12:00 UTC Dec 11 01:00–13:00 UTC Dec 12 02:00–11:00 UTC Dec 13 01:00–13:00 UTC Dec 14 01:00–13:00 UTC Dec 15 01:00–11:00 UTC Dec 16 01:00–11:00 UTC Dec 17 01:00–11:00 UTC Dec 18 01:00–12:00 UTC Dec 19 01:00–12:00 UTC Dec 20 01:00–16:00 UTC Dec 21 01:00–14:00 UTC Dec 22 01:00–10:00 UTC Dec 23 01:00–15:00 UTC Dec 24 01:00–12:00, 15:00–18:00 UTC Dec 25 01:00–17:00 UTC Dec 26 01:00–12:00 UTC Dec 27 01:00–10:00, 15:00–17:00 UTC Dec 28 01:00–13:00 UTC Dec 29 01:00–12:00, 15:00–17:00 UTC Dec 30 01:00–11:00, 15:00–18:00 UTC Dec 31 01:00–11:00 UTC