Most often in April, 25 days.
Highest follower count seen that year: 185,045.
January
Jan 1 05:00–12:00, 19:00–24:00 Jan 2 00:00–02:00, 21:00–22:00 Jan 3 08:00–12:00 Jan 4 09:00–12:00 Jan 6 06:00–10:00 Jan 8 08:00–12:00, 22:00–24:00 Jan 9 22:00–24:00 Jan 10 12:00–15:00, 22:00–24:00 Jan 11 00:00–01:00, 08:00–12:00 Jan 12 08:00–12:00, 21:00–24:00 Jan 14 08:00–10:00 Jan 15 10:00–12:00 Jan 17 08:00–10:00, 22:00–24:00 Jan 18 09:00–12:00 Jan 20 09:00–11:00, 21:00–24:00 Jan 21 09:00–12:00 Jan 22 04:00–08:00 Jan 23 08:00–12:00 Jan 24 05:00–09:00, 20:00–24:00 Jan 25 05:00–09:00, 20:00–21:00 Jan 26 05:00–09:00 Jan 30 08:00–12:00, 21:00–23:00 Jan 31 08:00–11:00
February
Feb 1 09:00–12:00 Feb 3 07:00–10:00 Feb 4 22:00–24:00 Feb 5 10:00–13:00 Feb 6 08:00–12:00, 22:00–23:00 Feb 7 00:00–01:00, 08:00–12:00 Feb 8 08:00–12:00, 19:00–21:00 Feb 10 07:00–10:00 Feb 11 20:00–22:00 Feb 12 10:00–14:00 Feb 13 08:00–10:00 Feb 14 08:00–12:00 Feb 15 08:00–12:00 Feb 17 16:00–19:00 Feb 18 08:00–11:00, 18:00–19:00 Feb 19 08:00–10:00 Feb 20 09:00–12:00 Feb 21 08:00–13:00 Feb 22 08:00–12:00, 18:00–21:00 Feb 23 08:00–11:00, 18:00–20:00 Feb 24 18:00–20:00 Feb 25 08:00–11:00 Feb 26 08:00–10:00 Feb 27 08:00–11:00, 17:00–19:00 Feb 28 08:00–12:00, 18:00–19:00
March
Mar 1 09:00–12:00 Mar 4 08:00–10:00 Mar 5 08:00–11:00 Mar 6 08:00–10:00, 18:00–20:00 Mar 7 08:00–12:00 Mar 8 08:00–12:00, 18:00–20:00 Mar 10 08:00–12:00, 19:00–23:00 Mar 11 08:00–10:00, 20:00–23:00 Mar 12 20:00–22:00 Mar 13 08:00–11:00 Mar 14 08:00–12:00, 21:00–23:00 Mar 15 07:00–12:00 Mar 16 20:00–24:00 Mar 18 21:00–24:00 Mar 19 18:00–22:00 Mar 20 08:00–11:00, 20:00–23:00 Mar 21 09:00–11:00, 19:00–23:00 Mar 22 08:00–12:00, 20:00–22:00 Mar 23 09:00–11:00 Mar 30 08:00–09:00, 19:00–23:00 Mar 31 18:00–20:00
April
Apr 3 08:00–11:00 Apr 4 07:00–10:00 Apr 5 07:00–10:00, 20:00–22:00 Apr 6 21:00–22:00 Apr 7 17:00–21:00 Apr 8 20:00–22:00 Apr 9 07:00–08:00, 22:00–24:00 Apr 10 08:00–11:00, 20:00–22:00 Apr 11 08:00–10:00, 20:00–24:00 Apr 12 00:00–02:00, 20:00–24:00 Apr 13 00:00–01:00, 20:00–23:00 Apr 14 20:00–23:00 Apr 15 08:00–11:00 Apr 18 09:00–10:00, 21:00–22:00 Apr 19 07:00–11:00, 19:00–23:00 Apr 20 20:00–24:00 Apr 22 20:00–24:00 Apr 23 08:00–11:00, 20:00–21:00 Apr 24 07:00–11:00, 20:00–21:00 Apr 25 07:00–10:00, 22:00–24:00 Apr 26 00:00–01:00, 08:00–11:00, 20:00–23:00 Apr 27 21:00–23:00 Apr 28 05:00–07:00, 20:00–21:00 Apr 29 08:00–10:00, 21:00–24:00 Apr 30 09:00–11:00
May
May 2 08:00–10:00 May 3 08:00–11:00, 19:00–21:00 May 5 07:00–11:00, 20:00–23:00 May 6 16:00–19:00 May 7 08:00–10:00, 17:00–20:00 May 8 08:00–10:00, 18:00–20:00 May 9 07:00–10:00 May 10 07:00–11:00, 17:00–22:00 May 11 18:00–20:00 May 12 18:00–19:00 May 13 08:00–10:00, 19:00–22:00 May 14 07:00–10:00 May 15 08:00–10:00, 16:00–18:00 May 16 07:00–11:00 May 17 08:00–12:00, 18:00–23:00 May 18 19:00–22:00 May 19 20:00–22:00 May 20 07:00–08:00, 20:00–22:00 May 21 08:00–10:00 May 22 08:00–11:00, 21:00–22:00 May 23 09:00–11:00
June
Jun 3 07:00–10:00, 19:00–21:00 Jun 6 07:00–11:00 Jun 7 07:00–11:00, 18:00–22:00 Jun 8 18:00–20:00 Jun 9 08:00–11:00, 18:00–20:00 Jun 10 06:00–08:00, 18:00–20:00, 22:00–24:00 Jun 11 17:00–21:00 Jun 12 07:00–10:00, 19:00–22:00 Jun 13 08:00–11:00 Jun 15 18:00–22:00 Jun 17 18:00–21:00 Jun 20 07:00–10:00, 18:00–21:00 Jun 21 07:00–10:00, 18:00–19:00 Jun 22 18:00–20:00 Jun 24 05:00–08:00, 17:00–20:00 Jun 25 06:00–07:00, 14:00–18:00 Jun 26 07:00–10:00, 18:00–20:00 Jun 27 06:00–08:00, 18:00–22:00 Jun 28 07:00–10:00, 18:00–21:00 Jun 29 08:00–10:00, 18:00–22:00 Jun 30 08:00–10:00, 16:00–18:00
July
Jul 3 17:00–20:00 Jul 4 07:00–10:00, 18:00–21:00 Jul 5 08:00–12:00, 18:00–19:00, 21:00–22:00 Jul 6 17:00–22:00 Jul 7 19:00–23:00 Jul 8 19:00–21:00 Jul 9 18:00–20:00 Jul 10 07:00–10:00, 20:00–23:00 Jul 11 08:00–10:00, 19:00–23:00 Jul 12 09:00–11:00, 18:00–22:00 Jul 13 20:00–24:00 Jul 14 19:00–23:00 Jul 15 19:00–22:00 Jul 16 21:00–22:00 Jul 18 09:00–11:00 Jul 19 08:00–11:00, 20:00–23:00 Jul 21 21:00–22:00 Jul 22 20:00–22:00 Jul 23 19:00–23:00 Jul 24 00:00–01:00, 16:00–18:00 Jul 25 09:00–11:00, 20:00–22:00 Jul 26 07:00–12:00, 19:00–23:00 Jul 27 19:00–21:00 Jul 30 21:00–22:00