Most often in April, 28 days.
Highest follower count seen that year: 16,365.
January
Jan 3 12:00–18:00 UTC Jan 4 10:00–13:00 UTC Jan 5 08:00–12:00, 14:00–16:00 UTC Jan 6 07:00–09:00 UTC Jan 7 08:00–11:00 UTC Jan 9 07:00–13:00, 19:00–22:00 UTC Jan 10 05:00–09:00 UTC Jan 12 10:00–17:00 UTC Jan 15 08:00–17:00 UTC Jan 16 07:00–14:00 UTC Jan 17 06:00–11:00 UTC Jan 18 16:00–19:00 UTC Jan 19 13:00–17:00 UTC Jan 21 10:00–16:00 UTC Jan 22 08:00–12:00 UTC Jan 23 08:00–17:00 UTC Jan 24 13:00–20:00 UTC Jan 25 08:00–12:00 UTC Jan 26 09:00–20:00 UTC Jan 27 07:00–14:00, 19:00–21:00 UTC Jan 28 10:00–22:00 UTC Jan 29 07:00–08:00, 11:00–15:00, 17:00–20:00 UTC Jan 30 07:00–13:00 UTC Jan 31 08:00–12:00 UTC
February
Feb 2 11:00–19:00 UTC Feb 3 06:00–14:00 UTC Feb 4 12:00–15:00 UTC Feb 5 15:00–18:00 UTC Feb 6 07:00–13:00 UTC Feb 8 07:00–10:00 UTC Feb 9 11:00–19:00 UTC Feb 10 07:00–13:00, 16:00–20:00 UTC Feb 11 10:00–18:00 UTC Feb 12 09:00–21:00 UTC Feb 13 07:00–13:00 UTC Feb 14 06:00–09:00 UTC Feb 15 08:00–17:00 UTC Feb 16 06:00–11:00, 13:00–18:00 UTC Feb 17 08:00–11:00 UTC Feb 18 12:00–13:00 UTC Feb 19 07:00–11:00, 15:00–19:00 UTC Feb 20 07:00–11:00, 13:00–19:00 UTC Feb 21 07:00–08:00 UTC Feb 22 06:00–12:00 UTC Feb 23 13:00–19:00 UTC Feb 24 10:00–19:00 UTC Feb 25 14:00–21:00 UTC Feb 26 07:00–13:00 UTC Feb 27 06:00–08:00, 10:00–15:00 UTC
March
Mar 3 07:00–19:00 UTC Mar 4 07:00–20:00 UTC Mar 5 04:00–20:00 UTC Mar 6 08:00–09:00 UTC Mar 9 12:00–19:00 UTC Mar 10 09:00–18:00 UTC Mar 11 06:00–16:00 UTC Mar 12 06:00–09:00, 14:00–19:00 UTC Mar 13 06:00–10:00 UTC Mar 14 06:00–09:00 UTC Mar 17 12:00–16:00 UTC Mar 18 11:00–19:00 UTC Mar 19 06:00–19:00 UTC Mar 20 05:00–07:00, 10:00–19:00 UTC Mar 21 05:00–18:00 UTC Mar 22 18:00–23:00 UTC Mar 23 09:00–20:00 UTC Mar 24 08:00–20:00 UTC Mar 25 07:00–09:00, 11:00–21:00 UTC Mar 26 07:00–10:00, 12:00–20:00 UTC Mar 27 06:00–13:00, 17:00–19:00 UTC Mar 28 19:00–22:00 UTC Mar 29 05:00–08:00, 11:00–20:00 UTC Mar 30 10:00–20:00 UTC Mar 31 05:00–08:00, 12:00–15:00 UTC
April
Apr 1 12:00–15:00, 18:00–21:00 UTC Apr 2 11:00–14:00 UTC Apr 3 10:00–17:00 UTC Apr 4 04:00–12:00 UTC Apr 5 13:00–16:00 UTC Apr 6 06:00–12:00 UTC Apr 7 10:00–20:00 UTC Apr 8 06:00–19:00 UTC Apr 9 04:00–17:00 UTC Apr 10 04:00–08:00 UTC Apr 12 14:00–21:00 UTC Apr 13 08:00–11:00, 14:00–19:00 UTC Apr 14 06:00–20:00 UTC Apr 15 11:00–17:00 UTC Apr 16 11:00–19:00 UTC Apr 17 18:00–21:00 UTC Apr 19 08:00–20:00 UTC Apr 20 07:00–21:00 UTC Apr 21 07:00–10:00, 12:00–13:00, 17:00–22:00 UTC Apr 22 05:00–09:00, 12:00–20:00 UTC Apr 23 10:00–20:00 UTC Apr 24 07:00–11:00, 13:00–17:00 UTC Apr 25 17:00–20:00 UTC Apr 26 05:00–07:00 UTC Apr 27 13:00–20:00 UTC Apr 28 08:00–09:00, 11:00–12:00, 15:00–19:00 UTC Apr 29 13:00–16:00 UTC Apr 30 09:00–16:00 UTC
May
May 3 07:00–12:00, 16:00–18:00 UTC May 4 07:00–08:00, 11:00–15:00 UTC May 5 10:00–17:00 UTC May 6 16:00–20:00 UTC May 7 08:00–11:00, 16:00–20:00 UTC May 8 05:00–08:00 UTC May 10 07:00–13:00 UTC May 11 05:00–13:00, 18:00–20:00 UTC May 12 10:00–12:00 UTC May 13 08:00–18:00 UTC May 14 08:00–17:00, 19:00–21:00 UTC May 15 10:00–13:00 UTC May 16 11:00–20:00 UTC May 17 08:00–16:00 UTC May 18 08:00–10:00 UTC May 20 08:00–16:00 UTC May 21 08:00–13:00, 16:00–19:00 UTC May 22 07:00–09:00 UTC May 23 06:00–12:00 UTC May 24 08:00–19:00 UTC May 25 06:00–18:00 UTC May 26 04:00–12:00 UTC May 28 16:00–18:00 UTC May 29 08:00–13:00 UTC May 30 14:00–19:00 UTC May 31 07:00–09:00 UTC
June
Jun 2 15:00–20:00 UTC Jun 3 08:00–16:00, 18:00–19:00 UTC Jun 4 08:00–13:00, 15:00–21:00 UTC Jun 5 05:00–11:00 UTC Jun 7 07:00–15:00 UTC Jun 8 14:00–20:00 UTC Jun 9 09:00–13:00, 17:00–20:00 UTC Jun 10 07:00–11:00 UTC Jun 11 09:00–14:00 UTC Jun 15 07:00–14:00 UTC Jun 17 11:00–19:00 UTC Jun 18 08:00–16:00 UTC Jun 19 14:00–17:00 UTC Jun 20 06:00–08:00, 15:00–17:00 UTC Jun 21 08:00–10:00, 15:00–18:00 UTC Jun 22 08:00–19:00 UTC Jun 23 17:00–20:00 UTC Jun 24 10:00–13:00 UTC Jun 25 03:00–09:00, 16:00–17:00 UTC Jun 26 07:00–11:00 UTC Jun 27 06:00–08:00 UTC Jun 28 15:00–18:00 UTC Jun 29 12:00–18:00 UTC Jun 30 15:00–16:00 UTC
July
Jul 1 06:00–10:00 UTC Jul 2 09:00–11:00 UTC Jul 3 09:00–11:00 UTC Jul 4 09:00–12:00 UTC Jul 5 15:00–19:00 UTC Jul 6 13:00–16:00 UTC Jul 8 09:00–11:00, 17:00–18:00 UTC Jul 9 05:00–06:00 UTC Jul 10 07:00–13:00 UTC Jul 11 07:00–11:00 UTC Jul 15 12:00–13:00 UTC Jul 17 14:00–17:00 UTC Jul 18 09:00–13:00 UTC Jul 19 10:00–19:00 UTC Jul 20 09:00–15:00 UTC Jul 21 09:00–12:00 UTC Jul 22 08:00–10:00, 12:00–21:00 UTC Jul 23 15:00–17:00 UTC Jul 24 05:00–10:00 UTC Jul 25 11:00–13:00 UTC Jul 26 13:00–16:00 UTC Jul 27 08:00–13:00 UTC