Most often in April, 28 days.
Highest follower count seen that year: 3,352.
January
Jan 3 11:00–17:00 UTC Jan 4 11:00–17:00 UTC Jan 5 11:00–17:00 UTC Jan 6 11:00–18:00 UTC Jan 7 11:00–17:00 UTC Jan 8 04:00–09:00 UTC Jan 9 10:00–17:00 UTC Jan 10 11:00–16:00 UTC Jan 11 11:00–13:00 UTC Jan 12 10:00–13:00 UTC Jan 13 11:00–18:00 UTC Jan 14 11:00–18:00 UTC Jan 16 10:00–17:00 UTC Jan 17 04:00–09:00 UTC Jan 18 10:00–16:00 UTC Jan 19 11:00–18:00 UTC Jan 20 11:00–17:00 UTC Jan 21 11:00–18:00 UTC Jan 22 11:00–18:00 UTC Jan 23 11:00–18:00 UTC Jan 24 11:00–17:00 UTC Jan 25 11:00–14:00, 16:00–17:00 UTC Jan 26 05:00–17:00 UTC Jan 27 05:00–07:00, 10:00–17:00 UTC Jan 28 05:00–06:00, 10:00–14:00, 16:00–18:00 UTC Jan 29 11:00–18:00 UTC Jan 30 11:00–18:00 UTC
February
Feb 1 11:00–16:00 UTC Feb 2 11:00–18:00 UTC Feb 9 13:00–18:00 UTC Feb 10 11:00–18:00 UTC Feb 11 10:00–13:00, 16:00–18:00 UTC Feb 12 11:00–18:00 UTC Feb 13 11:00–13:00, 15:00–18:00 UTC Feb 15 10:00–14:00 UTC Feb 16 11:00–18:00 UTC Feb 17 11:00–18:00 UTC Feb 18 12:00–19:00 UTC Feb 19 11:00–18:00 UTC Feb 20 11:00–17:00 UTC Feb 21 11:00–18:00 UTC Feb 22 11:00–21:00 UTC Feb 23 10:00–18:00 UTC Feb 24 11:00–15:00, 17:00–18:00 UTC Feb 25 11:00–18:00 UTC Feb 26 11:00–13:00, 15:00–18:00 UTC Feb 27 11:00–13:00, 15:00–18:00 UTC Feb 28 16:00–19:00 UTC
March
Mar 1 12:00–18:00 UTC Mar 2 10:00–18:00 UTC Mar 3 10:00–17:00 UTC Mar 4 10:00–18:00 UTC Mar 5 10:00–18:00 UTC Mar 6 10:00–18:00 UTC Mar 7 10:00–18:00 UTC Mar 8 10:00–17:00 UTC Mar 9 10:00–13:00, 15:00–18:00 UTC Mar 10 10:00–18:00 UTC Mar 11 12:00–13:00, 15:00–18:00 UTC Mar 17 11:00–12:00, 15:00–17:00 UTC Mar 18 11:00–18:00 UTC Mar 19 10:00–18:00 UTC Mar 20 11:00–20:00 UTC Mar 21 11:00–17:00 UTC Mar 22 11:00–18:00 UTC Mar 23 10:00–17:00 UTC Mar 24 11:00–18:00, 20:00–22:00 UTC Mar 25 11:00–18:00 UTC Mar 26 10:00–18:00 UTC Mar 27 11:00–18:00 UTC Mar 28 10:00–18:00 UTC Mar 29 11:00–18:00 UTC Mar 30 11:00–18:00 UTC Mar 31 06:00–07:00, 12:00–14:00, 16:00–17:00 UTC
April
Apr 2 11:00–18:00 UTC Apr 3 14:00–17:00 UTC Apr 4 11:00–17:00, 21:00–22:00 UTC Apr 5 11:00–17:00 UTC Apr 6 06:00–07:00, 11:00–18:00 UTC Apr 7 09:00–18:00 UTC Apr 8 10:00–18:00 UTC Apr 9 10:00–14:00, 16:00–17:00 UTC Apr 10 15:00–18:00 UTC Apr 11 11:00–19:00 UTC Apr 12 10:00–17:00 UTC Apr 13 11:00–19:00 UTC Apr 14 11:00–14:00, 16:00–19:00, 22:00–24:00 UTC Apr 15 00:00–01:00, 11:00–19:00 UTC Apr 16 00:00–01:00 UTC Apr 17 10:00–18:00 UTC Apr 18 11:00–19:00 UTC Apr 19 11:00–17:00 UTC Apr 20 10:00–18:00 UTC Apr 21 11:00–13:00 UTC Apr 22 11:00–16:00 UTC Apr 23 11:00–14:00 UTC Apr 24 11:00–17:00 UTC Apr 25 13:00–18:00 UTC Apr 26 10:00–18:00 UTC Apr 27 11:00–18:00 UTC Apr 28 10:00–18:00 UTC Apr 29 11:00–15:00 UTC
May
May 1 13:00–18:00, 21:00–23:00 UTC May 3 11:00–19:00 UTC May 4 11:00–17:00 UTC May 5 11:00–17:00 UTC May 6 11:00–13:00 UTC May 7 15:00–18:00 UTC May 8 12:00–15:00, 17:00–18:00 UTC May 9 10:00–13:00, 15:00–17:00 UTC May 10 10:00–12:00, 14:00–18:00 UTC May 11 10:00–17:00 UTC May 12 11:00–16:00 UTC May 13 10:00–13:00 UTC May 16 09:00–17:00 UTC May 17 10:00–16:00 UTC May 18 10:00–15:00 UTC May 19 10:00–18:00 UTC May 20 10:00–12:00, 15:00–17:00 UTC May 21 10:00–13:00, 16:00–17:00 UTC May 22 10:00–17:00 UTC May 23 12:00–18:00 UTC May 24 11:00–13:00, 15:00–16:00 UTC May 25 10:00–18:00 UTC May 26 11:00–17:00 UTC May 27 10:00–18:00 UTC May 28 10:00–15:00 UTC May 29 11:00–13:00 UTC May 30 11:00–13:00, 15:00–17:00 UTC
June
Jun 1 11:00–17:00 UTC Jun 2 12:00–17:00, 23:00–24:00 UTC Jun 3 00:00–02:00, 10:00–19:00 UTC Jun 5 12:00–18:00 UTC Jun 6 11:00–16:00 UTC Jun 7 11:00–18:00 UTC Jun 8 12:00–18:00 UTC Jun 9 11:00–16:00, 18:00–21:00, 23:00–24:00 UTC Jun 10 00:00–01:00, 08:00–19:00 UTC Jun 11 11:00–13:00, 20:00–21:00 UTC Jun 12 12:00–15:00, 23:00–24:00 UTC Jun 13 00:00–01:00, 11:00–18:00 UTC Jun 15 12:00–17:00, 22:00–23:00 UTC Jun 16 02:00–04:00 UTC Jun 17 11:00–12:00, 15:00–17:00 UTC Jun 18 11:00–17:00 UTC Jun 19 14:00–16:00 UTC Jun 20 11:00–17:00, 22:00–23:00 UTC Jun 21 10:00–17:00, 22:00–23:00 UTC Jun 22 10:00–19:00 UTC Jun 23 11:00–14:00, 16:00–17:00 UTC Jun 24 11:00–15:00 UTC Jun 25 10:00–15:00, 18:00–22:00 UTC Jun 26 10:00–18:00, 23:00–24:00 UTC Jun 27 00:00–01:00, 03:00–04:00 UTC Jun 28 10:00–16:00 UTC Jun 29 10:00–16:00 UTC Jun 30 15:00–17:00 UTC
July
Jul 1 11:00–12:00, 14:00–15:00 UTC Jul 2 10:00–16:00 UTC Jul 3 09:00–18:00, 23:00–24:00 UTC Jul 4 00:00–01:00, 12:00–16:00 UTC Jul 5 01:00–02:00, 10:00–19:00 UTC Jul 6 02:00–03:00, 12:00–15:00 UTC Jul 7 10:00–12:00, 17:00–18:00 UTC Jul 8 10:00–19:00, 23:00–24:00 UTC Jul 9 11:00–14:00 UTC Jul 10 10:00–17:00 UTC Jul 11 10:00–13:00, 17:00–18:00 UTC Jul 12 02:00–03:00, 10:00–12:00, 15:00–18:00 UTC Jul 13 10:00–18:00 UTC Jul 14 10:00–14:00, 16:00–18:00 UTC Jul 15 11:00–20:00 UTC Jul 16 02:00–03:00 UTC Jul 17 10:00–19:00 UTC Jul 18 11:00–17:00 UTC Jul 19 10:00–18:00 UTC Jul 20 12:00–17:00, 23:00–24:00 UTC Jul 21 01:00–03:00, 11:00–17:00, 19:00–22:00 UTC Jul 22 04:00–05:00, 11:00–19:00 UTC Jul 23 12:00–13:00, 15:00–24:00 UTC Jul 24 11:00–14:00 UTC Jul 25 00:00–03:00, 11:00–17:00 UTC Jul 26 12:00–15:00, 17:00–18:00, 21:00–22:00 UTC Jul 27 13:00–15:00, 17:00–18:00, 19:00–21:00 UTC