Most often in December, 29 days.
Highest follower count seen that year: 50,663.
July
Jul 29 13:00–20:00 UTC Jul 31 12:00–20:00 UTC
August
Aug 1 13:00–19:00 UTC Aug 2 14:00–20:00 UTC Aug 3 13:00–20:00 UTC Aug 5 12:00–20:00 UTC Aug 6 12:00–20:00 UTC Aug 9 12:00–20:00 UTC Aug 10 15:00–18:00 UTC Aug 12 12:00–20:00 UTC Aug 13 13:00–20:00 UTC Aug 14 13:00–20:00 UTC Aug 15 13:00–19:00 UTC Aug 17 13:00–20:00 UTC Aug 22 13:00–20:00 UTC Aug 23 13:00–20:00 UTC Aug 24 13:00–21:00 UTC Aug 25 13:00–20:00 UTC Aug 26 13:00–20:00 UTC Aug 27 13:00–20:00 UTC Aug 29 13:00–20:00 UTC Aug 30 13:00–21:00 UTC Aug 31 12:00–19:00 UTC
September
Sep 1 19:00–24:00 UTC Sep 2 00:00–03:00, 19:00–24:00 UTC Sep 3 00:00–03:00, 20:00–24:00 UTC Sep 4 00:00–02:00 UTC Sep 5 19:00–24:00 UTC Sep 6 00:00–03:00, 20:00–24:00 UTC Sep 7 00:00–03:00, 20:00–24:00 UTC Sep 8 00:00–03:00 UTC Sep 9 19:00–24:00 UTC Sep 10 00:00–03:00, 20:00–24:00 UTC Sep 11 00:00–03:00, 20:00–24:00 UTC Sep 12 00:00–03:00 UTC Sep 14 20:00–24:00 UTC Sep 15 00:00–03:00, 17:00–24:00 UTC Sep 16 19:00–24:00 UTC Sep 17 00:00–03:00, 19:00–24:00 UTC Sep 18 00:00–03:00 UTC Sep 19 20:00–24:00 UTC Sep 20 00:00–03:00, 20:00–22:00 UTC Sep 21 19:00–24:00 UTC Sep 22 00:00–03:00, 19:00–24:00 UTC Sep 23 00:00–03:00 UTC Sep 24 20:00–24:00 UTC Sep 25 00:00–03:00, 20:00–24:00 UTC Sep 26 00:00–03:00, 19:00–24:00 UTC Sep 27 00:00–03:00, 20:00–24:00 UTC Sep 28 00:00–04:00, 19:00–24:00 UTC Sep 29 00:00–04:00 UTC Sep 30 20:00–24:00 UTC
October
Oct 1 00:00–03:00, 19:00–24:00 UTC Oct 2 00:00–03:00 UTC Oct 4 20:00–24:00 UTC Oct 5 00:00–04:00, 20:00–24:00 UTC Oct 6 00:00–04:00, 20:00–24:00 UTC Oct 7 00:00–04:00, 19:00–24:00 UTC Oct 8 00:00–04:00, 19:00–24:00 UTC Oct 9 00:00–03:00 UTC Oct 14 20:00–24:00 UTC Oct 15 00:00–03:00, 20:00–24:00 UTC Oct 16 00:00–04:00, 20:00–24:00 UTC Oct 17 00:00–03:00, 20:00–24:00 UTC Oct 18 00:00–03:00, 20:00–24:00 UTC Oct 19 00:00–01:00, 19:00–24:00 UTC Oct 20 00:00–03:00, 19:00–24:00 UTC Oct 21 00:00–03:00 UTC Oct 22 20:00–24:00 UTC Oct 23 00:00–03:00 UTC Oct 24 19:00–24:00 UTC Oct 25 00:00–03:00, 19:00–24:00 UTC Oct 26 00:00–04:00, 21:00–24:00 UTC Oct 27 00:00–04:00, 21:00–24:00 UTC Oct 28 00:00–04:00 UTC Oct 29 20:00–24:00 UTC Oct 30 00:00–04:00, 20:00–24:00 UTC Oct 31 00:00–04:00 UTC
November
Nov 2 21:00–24:00 UTC Nov 3 00:00–04:00, 20:00–24:00 UTC Nov 4 00:00–04:00, 20:00–24:00 UTC Nov 5 00:00–04:00, 20:00–24:00 UTC Nov 6 00:00–04:00, 20:00–24:00 UTC Nov 7 00:00–05:00 UTC Nov 8 21:00–24:00 UTC Nov 9 00:00–04:00 UTC Nov 10 20:00–24:00 UTC Nov 11 00:00–04:00, 21:00–24:00 UTC Nov 12 00:00–04:00, 21:00–24:00 UTC Nov 13 00:00–04:00 UTC Nov 15 21:00–24:00 UTC Nov 16 00:00–04:00, 21:00–24:00 UTC Nov 17 00:00–04:00, 21:00–24:00 UTC Nov 18 00:00–02:00 UTC Nov 24 21:00–24:00 UTC Nov 25 00:00–04:00 UTC Nov 26 21:00–24:00 UTC Nov 27 00:00–04:00 UTC Nov 28 19:00–24:00 UTC Nov 29 00:00–03:00, 19:00–24:00 UTC Nov 30 00:00–04:00, 19:00–24:00 UTC
December
Dec 1 00:00–03:00, 19:00–24:00 UTC Dec 2 00:00–03:00 UTC Dec 3 20:00–24:00 UTC Dec 4 00:00–04:00, 21:00–24:00 UTC Dec 5 00:00–04:00, 19:00–24:00 UTC Dec 6 00:00–02:00 UTC Dec 8 21:00–24:00 UTC Dec 9 00:00–05:00, 21:00–24:00 UTC Dec 10 00:00–05:00, 21:00–24:00 UTC Dec 11 00:00–05:00, 20:00–24:00 UTC Dec 12 00:00–04:00, 20:00–24:00 UTC Dec 13 00:00–04:00, 20:00–24:00 UTC Dec 14 00:00–04:00, 20:00–24:00 UTC Dec 15 00:00–04:00, 20:00–24:00 UTC Dec 16 00:00–04:00 UTC Dec 17 20:00–24:00 UTC Dec 18 00:00–04:00, 20:00–24:00 UTC Dec 19 00:00–04:00, 22:00–24:00 UTC Dec 20 00:00–04:00, 20:00–24:00 UTC Dec 21 00:00–04:00, 20:00–24:00 UTC Dec 22 00:00–04:00, 21:00–24:00 UTC Dec 23 00:00–04:00, 20:00–24:00 UTC Dec 24 00:00–03:00 UTC Dec 26 20:00–24:00 UTC Dec 27 00:00–03:00, 20:00–24:00 UTC Dec 28 00:00–04:00, 20:00–24:00 UTC Dec 29 00:00–03:00, 20:00–24:00 UTC Dec 30 00:00–04:00, 21:00–24:00 UTC Dec 31 00:00–04:00 UTC