Most often in May, 30 days.
Highest follower count seen that year: 38,377.
January
Jan 1 04:00–19:00 Jan 2 12:00–17:00 Jan 3 02:00–06:00 Jan 4 03:00–10:00 Jan 5 04:00–08:00 Jan 6 04:00–12:00 Jan 7 04:00–10:00 Jan 8 04:00–09:00, 12:00–16:00 Jan 9 03:00–08:00, 13:00–16:00 Jan 10 03:00–07:00 Jan 11 02:00–08:00, 11:00–15:00 Jan 12 02:00–08:00, 12:00–15:00 Jan 13 06:00–10:00 Jan 14 12:00–16:00 Jan 15 05:00–11:00 Jan 16 04:00–10:00 Jan 17 04:00–08:00, 15:00–17:00 Jan 18 04:00–07:00 Jan 19 02:00–05:00, 12:00–14:00 Jan 20 03:00–05:00 Jan 21 05:00–11:00, 15:00–18:00 Jan 22 03:00–07:00, 15:00–18:00 Jan 23 06:00–11:00 Jan 24 07:00–10:00 Jan 25 04:00–09:00, 12:00–16:00 Jan 26 02:00–07:00, 14:00–17:00 Jan 27 06:00–10:00
February
Feb 15 05:00–08:00 Feb 16 07:00–12:00 Feb 17 03:00–10:00 Feb 21 19:00–21:00 Feb 22 09:00–11:00, 13:00–16:00 Feb 23 08:00–14:00 Feb 24 05:00–09:00, 13:00–17:00 Feb 25 05:00–12:00, 19:00–20:00 Feb 26 04:00–13:00, 16:00–17:00 Feb 27 06:00–12:00, 19:00–20:00 Feb 28 05:00–10:00, 12:00–15:00
March
Mar 1 09:00–13:00, 16:00–19:00 Mar 2 06:00–09:00, 12:00–16:00 Mar 3 06:00–10:00 Mar 5 17:00–20:00 Mar 6 19:00–20:00 Mar 7 06:00–09:00 Mar 14 09:00–14:00 Mar 15 03:00–10:00, 12:00–14:00 Mar 16 10:00–20:00 Mar 17 05:00–11:00, 14:00–17:00 Mar 18 03:00–10:00, 15:00–18:00 Mar 19 04:00–07:00, 13:00–15:00 Mar 20 05:00–12:00 Mar 21 08:00–15:00, 17:00–19:00 Mar 22 05:00–11:00, 15:00–19:00 Mar 23 05:00–11:00, 15:00–19:00 Mar 24 06:00–10:00 Mar 28 20:00–22:00 Mar 29 21:00–22:00 Mar 30 06:00–09:00 Mar 31 16:00–20:00
April
Apr 1 05:00–07:00 Apr 4 08:00–15:00 Apr 5 04:00–11:00, 14:00–18:00 Apr 6 04:00–09:00, 12:00–16:00 Apr 7 03:00–14:00 Apr 8 05:00–15:00 Apr 9 04:00–14:00 Apr 10 04:00–10:00, 12:00–15:00 Apr 11 05:00–13:00 Apr 12 05:00–11:00, 13:00–17:00 Apr 13 02:00–09:00, 12:00–15:00 Apr 14 04:00–10:00 Apr 15 02:00–09:00, 13:00–16:00 Apr 16 02:00–08:00, 13:00–14:00 Apr 17 08:00–11:00 Apr 18 02:00–15:00 Apr 19 03:00–08:00, 11:00–15:00 Apr 20 03:00–11:00 Apr 21 03:00–09:00, 14:00–15:00 Apr 22 02:00–08:00, 13:00–16:00 Apr 23 03:00–10:00, 13:00–16:00 Apr 24 03:00–10:00, 13:00–16:00 Apr 25 02:00–10:00, 12:00–15:00 Apr 26 03:00–09:00, 12:00–15:00 Apr 27 03:00–10:00, 12:00–16:00 Apr 28 14:00–18:00 Apr 29 03:00–10:00, 14:00–17:00 Apr 30 04:00–11:00, 14:00–17:00
May
May 1 13:00–16:00, 18:00–21:00 May 2 03:00–07:00, 10:00–12:00 May 3 04:00–10:00, 14:00–17:00 May 4 05:00–10:00, 14:00–15:00 May 5 03:00–05:00, 07:00–10:00, 14:00–17:00 May 6 03:00–10:00, 19:00–20:00 May 7 03:00–11:00, 14:00–17:00 May 8 07:00–12:00 May 9 03:00–10:00, 15:00–17:00 May 10 03:00–06:00, 09:00–11:00, 19:00–22:00 May 11 04:00–10:00, 13:00–17:00 May 12 15:00–17:00 May 13 03:00–09:00, 13:00–17:00 May 14 04:00–10:00, 14:00–20:00 May 15 04:00–11:00 May 16 08:00–12:00, 16:00–18:00 May 17 05:00–16:00 May 18 04:00–10:00, 15:00–17:00 May 19 07:00–11:00 May 20 06:00–10:00, 15:00–18:00 May 21 07:00–10:00, 16:00–17:00 May 22 02:00–08:00, 13:00–16:00 May 23 04:00–10:00, 14:00–17:00 May 24 04:00–10:00, 13:00–16:00 May 25 02:00–08:00, 14:00–16:00 May 26 07:00–11:00 May 27 04:00–10:00 May 28 05:00–10:00, 17:00–19:00 May 29 03:00–11:00 May 30 04:00–06:00
June
Jun 22 06:00–07:00 Jun 23 06:00–08:00 Jun 24 07:00–14:00 Jun 25 03:00–04:00 Jun 27 05:00–09:00, 13:00–15:00 Jun 28 03:00–05:00, 13:00–15:00 Jun 29 04:00–05:00 Jun 30 03:00–07:00
July
Jul 1 06:00–10:00 Jul 3 05:00–09:00 Jul 4 02:00–07:00 Jul 5 03:00–07:00, 11:00–15:00 Jul 6 02:00–08:00, 10:00–13:00 Jul 7 03:00–09:00 Jul 8 14:00–15:00 Jul 9 02:00–09:00 Jul 10 02:00–07:00, 09:00–11:00 Jul 11 04:00–10:00 Jul 12 03:00–09:00 Jul 13 03:00–09:00 Jul 14 06:00–11:00 Jul 15 07:00–09:00 Jul 16 04:00–10:00 Jul 18 07:00–10:00, 13:00–16:00 Jul 19 03:00–11:00 Jul 20 04:00–12:00 Jul 24 05:00–09:00 Jul 25 02:00–05:00, 12:00–15:00 Jul 26 03:00–06:00, 07:00–11:00, 23:00–24:00 Jul 27 00:00–02:00, 05:00–07:00, 10:00–12:00, 13:00–14:00