Most often in April, 27 days.
Highest follower count seen that year: 507,302.
January
Jan 7 02:00–10:00 Jan 8 03:00–04:00, 06:00–08:00 Jan 9 02:00–10:00 Jan 10 02:00–11:00 Jan 14 02:00–09:00 Jan 15 02:00–11:00 Jan 16 02:00–11:00 Jan 17 02:00–04:00, 06:00–08:00, 17:00–21:00 Jan 18 02:00–06:00 Jan 19 02:00–10:00 Jan 30 02:00–04:00 Jan 31 02:00–11:00
February
Feb 3 02:00–11:00 Feb 4 02:00–11:00 Feb 5 02:00–11:00 Feb 6 02:00–06:00 Feb 11 02:00–06:00 Feb 12 02:00–05:00 Feb 14 02:00–10:00 Feb 15 02:00–11:00 Feb 16 02:00–12:00 Feb 17 02:00–11:00 Feb 19 02:00–10:00 Feb 20 02:00–11:00 Feb 21 02:00–03:00, 07:00–12:00 Feb 22 03:00–13:00 Feb 25 04:00–13:00 Feb 26 02:00–11:00 Feb 27 04:00–14:00 Feb 28 03:00–11:00
March
Mar 1 04:00–13:00 Mar 4 04:00–14:00 Mar 5 04:00–07:00 Mar 6 06:00–12:00 Mar 7 06:00–13:00 Mar 11 06:00–11:00 Mar 12 06:00–14:00 Mar 13 05:00–14:00 Mar 14 04:00–14:00 Mar 15 06:00–15:00 Mar 18 04:00–13:00 Mar 19 15:00–24:00 Mar 20 15:00–24:00 Mar 24 16:00–24:00 Mar 25 16:00–24:00 Mar 26 00:00–01:00, 16:00–24:00 Mar 29 15:00–23:00 Mar 30 15:00–17:00
April
Apr 1 16:00–24:00 Apr 2 00:00–01:00, 17:00–24:00 Apr 3 00:00–01:00, 16:00–17:00 Apr 4 16:00–18:00 Apr 8 16:00–24:00 Apr 9 16:00–24:00 Apr 10 00:00–01:00, 17:00–24:00 Apr 11 00:00–01:00, 16:00–24:00 Apr 12 16:00–24:00 Apr 13 15:00–23:00 Apr 14 15:00–23:00 Apr 15 16:00–24:00 Apr 16 16:00–23:00 Apr 17 16:00–20:00 Apr 18 16:00–24:00 Apr 19 00:00–01:00, 16:00–24:00 Apr 20 15:00–23:00 Apr 21 15:00–23:00 Apr 22 15:00–21:00 Apr 23 02:00–04:00, 15:00–22:00 Apr 24 02:00–07:00, 09:00–12:00, 15:00–23:00 Apr 25 02:00–12:00 Apr 26 15:00–23:00 Apr 27 02:00–11:00, 15:00–23:00 Apr 28 15:00–19:00 Apr 29 07:00–10:00, 15:00–22:00 Apr 30 02:00–05:00, 15:00–21:00
May
May 1 02:00–12:00 May 2 02:00–12:00 May 3 02:00–11:00, 15:00–23:00 May 4 02:00–11:00, 15:00–21:00 May 5 14:00–21:00 May 7 14:00–21:00 May 8 15:00–21:00 May 10 15:00–21:00 May 11 15:00–21:00 May 12 14:00–21:00 May 13 21:00–24:00 May 14 00:00–03:00, 15:00–21:00 May 15 22:00–24:00 May 16 00:00–13:00 May 17 02:00–12:00 May 18 15:00–23:00 May 19 15:00–22:00 May 20 15:00–19:00 May 21 02:00–11:00, 14:00–19:00 May 22 02:00–11:00 May 24 02:00–12:00, 16:00–22:00 May 25 02:00–11:00 May 28 02:00–12:00, 17:00–19:00 May 29 02:00–19:00
June
Jun 1 02:00–11:00 Jun 2 02:00–03:00, 13:00–19:00 Jun 3 13:00–19:00 Jun 4 13:00–19:00 Jun 5 02:00–18:00 Jun 6 02:00–19:00 Jun 7 04:00–12:00 Jun 9 02:00–12:00 Jun 10 13:00–19:00 Jun 11 13:00–19:00 Jun 12 13:00–19:00 Jun 13 13:00–17:00 Jun 16 02:00–10:00 Jun 17 02:00–12:00 Jun 22 09:00–15:00 Jun 23 14:00–20:00 Jun 24 14:00–20:00 Jun 26 14:00–21:00 Jun 27 15:00–20:00 Jun 30 14:00–20:00
July
Jul 1 14:00–24:00 Jul 2 00:00–06:00, 14:00–20:00 Jul 3 03:00–10:00, 14:00–20:00 Jul 4 14:00–18:00 Jul 8 15:00–20:00 Jul 9 02:00–05:00, 07:00–10:00, 14:00–20:00 Jul 10 03:00–10:00, 14:00–20:00 Jul 11 14:00–20:00 Jul 12 03:00–08:00 Jul 14 14:00–17:00 Jul 15 04:00–10:00 Jul 16 04:00–07:00 Jul 17 04:00–09:00 Jul 18 03:00–11:00 Jul 21 15:00–20:00 Jul 22 14:00–20:00 Jul 23 03:00–11:00, 14:00–20:00 Jul 24 14:00–18:00 Jul 25 04:00–10:00, 12:00–20:00 Jul 26 02:00–09:00