Most often in January, 29 days.
Highest follower count seen that year: 1,856.
January
Jan 3 00:00–03:00, 09:00–12:00 UTC Jan 4 00:00–03:00, 17:00–24:00 UTC Jan 5 00:00–02:00, 19:00–24:00 UTC Jan 6 00:00–01:00, 17:00–18:00, 20:00–24:00 UTC Jan 7 00:00–01:00, 18:00–24:00 UTC Jan 8 00:00–01:00, 18:00–21:00 UTC Jan 9 17:00–24:00 UTC Jan 10 00:00–03:00, 12:00–17:00 UTC Jan 11 00:00–04:00, 19:00–24:00 UTC Jan 12 18:00–20:00 UTC Jan 13 18:00–21:00 UTC Jan 14 18:00–24:00 UTC Jan 15 18:00–22:00 UTC Jan 16 18:00–24:00 UTC Jan 17 15:00–18:00, 22:00–24:00 UTC Jan 18 00:00–01:00, 12:00–15:00, 20:00–23:00 UTC Jan 19 19:00–23:00 UTC Jan 20 23:00–24:00 UTC Jan 21 19:00–23:00 UTC Jan 22 00:00–01:00, 19:00–23:00 UTC Jan 23 23:00–24:00 UTC Jan 24 00:00–01:00, 17:00–23:00 UTC Jan 25 12:00–14:00 UTC Jan 26 19:00–24:00 UTC Jan 27 00:00–01:00 UTC Jan 28 20:00–22:00 UTC Jan 29 19:00–24:00 UTC Jan 30 18:00–24:00 UTC Jan 31 00:00–02:00, 12:00–14:00, 21:00–24:00 UTC
February
Feb 1 00:00–01:00, 13:00–14:00, 17:00–20:00, 22:00–24:00 UTC Feb 2 00:00–01:00, 15:00–18:00, 21:00–22:00 UTC Feb 3 00:00–03:00, 20:00–22:00 UTC Feb 4 01:00–02:00, 15:00–18:00 UTC Feb 5 17:00–20:00 UTC Feb 6 19:00–24:00 UTC Feb 7 19:00–20:00 UTC Feb 8 01:00–02:00, 17:00–23:00 UTC Feb 9 20:00–22:00 UTC Feb 10 20:00–24:00 UTC Feb 11 21:00–24:00 UTC Feb 12 18:00–24:00 UTC Feb 13 18:00–19:00 UTC Feb 14 00:00–02:00, 14:00–15:00 UTC Feb 15 00:00–03:00, 14:00–17:00, 19:00–22:00 UTC Feb 18 19:00–21:00 UTC Feb 19 19:00–22:00 UTC Feb 20 00:00–01:00, 19:00–22:00 UTC Feb 21 17:00–18:00 UTC Feb 24 20:00–23:00 UTC Feb 25 19:00–21:00 UTC Feb 27 20:00–24:00 UTC Feb 28 15:00–18:00, 20:00–24:00 UTC
March
Mar 1 00:00–01:00, 15:00–17:00, 20:00–23:00 UTC Mar 2 13:00–16:00, 20:00–23:00 UTC Mar 3 08:00–10:00 UTC Mar 4 21:00–22:00 UTC Mar 5 08:00–13:00 UTC Mar 6 17:00–19:00, 23:00–24:00 UTC Mar 7 00:00–01:00, 15:00–16:00 UTC Mar 8 03:00–04:00 UTC Mar 10 19:00–22:00 UTC Mar 12 21:00–22:00 UTC Mar 13 21:00–24:00 UTC Mar 14 00:00–01:00, 11:00–12:00, 21:00–24:00 UTC Mar 15 00:00–05:00, 14:00–16:00 UTC Mar 16 20:00–22:00 UTC Mar 17 21:00–22:00 UTC Mar 18 15:00–19:00 UTC Mar 19 20:00–21:00 UTC Mar 20 00:00–01:00, 18:00–24:00 UTC Mar 21 00:00–01:00, 03:00–04:00, 21:00–24:00 UTC Mar 22 00:00–05:00, 12:00–13:00, 17:00–18:00 UTC Mar 24 19:00–22:00 UTC Mar 25 20:00–24:00 UTC Mar 26 00:00–01:00, 19:00–21:00, 23:00–24:00 UTC Mar 27 00:00–03:00, 12:00–14:00, 19:00–20:00 UTC Mar 28 13:00–14:00, 19:00–21:00 UTC Mar 29 00:00–02:00, 14:00–15:00, 19:00–20:00 UTC Mar 30 12:00–14:00, 19:00–24:00 UTC Mar 31 13:00–14:00, 23:00–24:00 UTC
April
Apr 1 00:00–01:00, 18:00–21:00 UTC Apr 2 22:00–24:00 UTC Apr 3 00:00–02:00, 14:00–15:00 UTC Apr 4 19:00–24:00 UTC Apr 5 00:00–03:00, 17:00–21:00 UTC Apr 6 13:00–14:00, 17:00–19:00 UTC Apr 10 17:00–18:00 UTC Apr 11 03:00–06:00, 13:00–14:00, 20:00–24:00 UTC Apr 12 00:00–02:00, 14:00–15:00, 21:00–23:00 UTC Apr 13 21:00–23:00 UTC Apr 14 15:00–16:00, 19:00–20:00, 22:00–23:00 UTC Apr 16 19:00–23:00 UTC Apr 17 14:00–15:00, 23:00–24:00 UTC Apr 18 00:00–06:00, 08:00–11:00, 18:00–23:00 UTC Apr 19 01:00–02:00, 12:00–13:00, 15:00–16:00, 19:00–22:00 UTC Apr 20 16:00–18:00, 20:00–22:00 UTC Apr 21 17:00–24:00 UTC Apr 22 07:00–12:00, 19:00–22:00 UTC Apr 23 18:00–23:00 UTC Apr 24 17:00–20:00, 22:00–24:00 UTC Apr 25 00:00–02:00, 18:00–23:00 UTC Apr 26 00:00–13:00 UTC Apr 27 19:00–21:00 UTC Apr 28 19:00–22:00 UTC Apr 29 19:00–21:00 UTC Apr 30 18:00–23:00 UTC
May
May 1 19:00–21:00 UTC May 2 00:00–02:00, 19:00–20:00 UTC May 3 11:00–12:00, 21:00–23:00 UTC May 4 12:00–16:00, 19:00–21:00 UTC May 8 20:00–24:00 UTC May 9 00:00–01:00, 06:00–07:00, 22:00–24:00 UTC May 10 00:00–01:00, 05:00–06:00, 16:00–17:00 UTC May 12 19:00–24:00 UTC May 13 19:00–23:00 UTC May 14 16:00–18:00, 22:00–23:00 UTC May 15 19:00–20:00 UTC May 16 05:00–06:00, 12:00–14:00, 23:00–24:00 UTC May 17 11:00–12:00, 18:00–22:00 UTC May 18 17:00–22:00 UTC May 19 18:00–23:00 UTC May 20 18:00–22:00 UTC May 21 19:00–22:00 UTC May 22 20:00–23:00 UTC May 23 01:00–02:00, 05:00–10:00, 15:00–17:00 UTC May 24 20:00–24:00 UTC May 25 00:00–04:00, 13:00–14:00 UTC May 26 17:00–19:00 UTC May 28 17:00–19:00 UTC May 30 00:00–02:00, 23:00–24:00 UTC May 31 22:00–24:00 UTC
June
Jun 1 00:00–01:00, 18:00–22:00 UTC Jun 2 17:00–24:00 UTC Jun 3 00:00–01:00 UTC Jun 4 18:00–22:00 UTC Jun 5 18:00–24:00 UTC Jun 6 01:00–02:00, 14:00–15:00, 18:00–21:00 UTC Jun 7 00:00–03:00, 21:00–22:00 UTC Jun 8 21:00–23:00 UTC Jun 9 18:00–23:00 UTC Jun 10 20:00–24:00 UTC Jun 11 00:00–01:00, 22:00–23:00 UTC Jun 12 22:00–24:00 UTC Jun 13 02:00–03:00, 16:00–17:00 UTC Jun 15 18:00–22:00 UTC Jun 16 18:00–21:00 UTC Jun 18 21:00–23:00 UTC Jun 19 23:00–24:00 UTC Jun 20 00:00–01:00, 22:00–24:00 UTC Jun 21 00:00–01:00, 21:00–22:00 UTC Jun 23 19:00–20:00, 22:00–24:00 UTC Jun 25 19:00–24:00 UTC Jun 26 20:00–24:00 UTC Jun 27 00:00–01:00, 15:00–18:00, 23:00–24:00 UTC Jun 29 20:00–24:00 UTC Jun 30 23:00–24:00 UTC
July
Jul 1 02:00–03:00 UTC