Most often in March, 30 days.
Highest follower count seen that year: 26,814.
January
Jan 2 09:00–12:00 Jan 3 18:00–19:00 Jan 4 03:00–04:00, 11:00–12:00 Jan 5 10:00–12:00 Jan 6 10:00–12:00 Jan 7 00:00–02:00 Jan 9 23:00–24:00 Jan 10 00:00–03:00, 18:00–20:00 Jan 12 10:00–12:00 Jan 13 17:00–19:00 Jan 15 23:00–24:00 Jan 16 00:00–01:00, 23:00–24:00 Jan 17 00:00–01:00 Jan 18 17:00–20:00 Jan 19 00:00–02:00 Jan 20 00:00–01:00, 18:00–19:00, 23:00–24:00 Jan 21 00:00–01:00, 23:00–24:00 Jan 22 00:00–02:00, 17:00–19:00, 23:00–24:00 Jan 23 00:00–01:00 Jan 24 00:00–01:00, 18:00–19:00 Jan 27 17:00–19:00, 22:00–24:00 Jan 29 18:00–22:00 Jan 31 03:00–05:00
February
Feb 1 00:00–03:00 Feb 4 10:00–11:00 Feb 5 18:00–20:00 Feb 6 03:00–05:00 Feb 7 16:00–17:00, 23:00–24:00 Feb 8 00:00–02:00 Feb 9 10:00–12:00 Feb 10 17:00–19:00, 22:00–24:00 Feb 11 00:00–01:00 Feb 12 16:00–19:00, 23:00–24:00 Feb 14 00:00–03:00, 17:00–19:00 Feb 15 00:00–02:00, 22:00–24:00 Feb 16 00:00–01:00 Feb 17 18:00–21:00 Feb 18 23:00–24:00 Feb 19 00:00–01:00 Feb 20 23:00–24:00 Feb 21 00:00–02:00, 22:00–24:00 Feb 22 00:00–01:00, 23:00–24:00 Feb 23 00:00–01:00, 22:00–24:00 Feb 24 17:00–19:00, 23:00–24:00 Feb 26 00:00–01:00, 18:00–20:00, 23:00–24:00 Feb 27 00:00–01:00, 23:00–24:00 Feb 28 00:00–02:00, 18:00–19:00
March
Mar 1 02:00–03:00 Mar 2 00:00–02:00 Mar 3 23:00–24:00 Mar 4 00:00–01:00 Mar 5 00:00–02:00, 18:00–19:00 Mar 6 00:00–03:00 Mar 7 01:00–03:00, 17:00–19:00 Mar 8 01:00–02:00, 17:00–18:00 Mar 9 00:00–01:00 Mar 10 23:00–24:00 Mar 11 10:00–12:00 Mar 12 18:00–19:00 Mar 13 04:00–05:00 Mar 14 09:00–11:00, 17:00–18:00 Mar 15 04:00–06:00 Mar 16 10:00–11:00 Mar 17 17:00–18:00, 23:00–24:00 Mar 18 00:00–01:00, 10:00–13:00 Mar 19 17:00–19:00, 23:00–24:00 Mar 20 23:00–24:00 Mar 21 22:00–24:00 Mar 22 00:00–02:00 Mar 23 23:00–24:00 Mar 24 23:00–24:00 Mar 25 00:00–02:00, 23:00–24:00 Mar 26 00:00–01:00, 18:00–20:00 Mar 27 01:00–02:00 Mar 28 17:00–18:00 Mar 29 03:00–04:00 Mar 31 17:00–19:00
April
Apr 2 22:00–24:00 Apr 5 00:00–01:00 Apr 6 00:00–02:00 Apr 7 02:00–04:00, 16:00–18:00 Apr 8 02:00–04:00 Apr 10 00:00–03:00, 09:00–11:00 Apr 12 03:00–06:00 Apr 13 08:00–09:00, 23:00–24:00 Apr 14 00:00–02:00, 16:00–19:00 Apr 15 09:00–10:00 Apr 16 08:00–11:00, 23:00–24:00 Apr 18 00:00–02:00 Apr 19 22:00–24:00 Apr 20 00:00–02:00, 14:00–16:00 Apr 21 17:00–19:00, 23:00–24:00 Apr 22 00:00–03:00, 23:00–24:00 Apr 23 00:00–01:00, 16:00–18:00, 22:00–24:00 Apr 24 00:00–01:00, 23:00–24:00 Apr 25 00:00–01:00, 16:00–18:00 Apr 26 00:00–01:00 Apr 27 00:00–01:00 Apr 28 17:00–19:00, 21:00–24:00 Apr 30 17:00–18:00, 22:00–24:00
May
May 2 00:00–03:00 May 3 00:00–02:00, 23:00–24:00 May 4 00:00–02:00 May 6 01:00–03:00, 23:00–24:00 May 7 00:00–01:00 May 8 02:00–03:00 May 9 17:00–18:00 May 11 00:00–02:00 May 12 17:00–19:00 May 14 03:00–05:00, 17:00–18:00, 20:00–21:00 May 16 00:00–01:00 May 17 05:00–08:00 May 19 17:00–18:00 May 21 17:00–18:00, 22:00–24:00 May 22 00:00–01:00 May 23 16:00–18:00 May 24 08:00–10:00 May 25 21:00–23:00 May 26 17:00–19:00, 21:00–23:00 May 27 23:00–24:00 May 28 16:00–18:00, 23:00–24:00 May 29 00:00–01:00, 22:00–24:00 May 30 00:00–01:00 May 31 16:00–19:00, 23:00–24:00
June
Jun 1 00:00–01:00, 23:00–24:00 Jun 2 00:00–03:00, 17:00–18:00, 23:00–24:00 Jun 3 00:00–01:00 Jun 4 00:00–02:00, 18:00–19:00, 23:00–24:00 Jun 5 00:00–01:00, 23:00–24:00 Jun 6 00:00–02:00, 17:00–19:00 Jun 7 01:00–04:00 Jun 9 17:00–18:00 Jun 10 01:00–04:00 Jun 11 00:00–03:00, 18:00–19:00, 22:00–24:00 Jun 12 23:00–24:00 Jun 13 00:00–03:00 Jun 15 01:00–03:00 Jun 16 15:00–17:00 Jun 17 01:00–03:00 Jun 18 16:00–17:00, 21:00–24:00 Jun 19 22:00–24:00 Jun 21 16:00–19:00 Jun 22 07:00–10:00, 23:00–24:00 Jun 23 00:00–01:00, 17:00–18:00 Jun 24 00:00–02:00, 23:00–24:00 Jun 25 00:00–01:00 Jun 26 23:00–24:00 Jun 27 00:00–01:00, 16:00–17:00 Jun 28 16:00–18:00 Jun 29 00:00–02:00, 23:00–24:00 Jun 30 16:00–17:00
July
Jul 1 00:00–03:00, 23:00–24:00 Jul 2 16:00–19:00 Jul 3 00:00–02:00 Jul 4 01:00–03:00 Jul 7 17:00–18:00 Jul 8 22:00–24:00 Jul 9 00:00–01:00, 17:00–18:00 Jul 10 00:00–02:00 Jul 11 00:00–02:00, 16:00–18:00 Jul 12 22:00–24:00 Jul 13 23:00–24:00 Jul 14 00:00–01:00, 23:00–24:00 Jul 15 00:00–01:00 Jul 21 17:00–18:00 Jul 22 04:00–05:00 Jul 24 17:00–18:00 Jul 25 16:00–18:00 Jul 30 17:00–18:00, 22:00–24:00