Most often in May, 31 days.
Highest follower count seen that year: 41,548.
March
Mar 16 18:00–20:00 UTC Mar 17 11:00–12:00 UTC Mar 18 12:00–14:00 UTC Mar 19 12:00–14:00 UTC Mar 20 20:00–22:00 UTC Mar 21 12:00–15:00 UTC Mar 22 11:00–15:00 UTC Mar 24 05:00–06:00 UTC Mar 25 06:00–08:00, 13:00–15:00 UTC Mar 26 13:00–15:00 UTC Mar 28 12:00–13:00 UTC Mar 29 06:00–07:00, 15:00–18:00 UTC Mar 30 11:00–15:00 UTC
April
Apr 1 12:00–17:00 UTC Apr 2 11:00–14:00 UTC Apr 3 05:00–07:00, 13:00–15:00 UTC Apr 4 14:00–16:00 UTC Apr 5 14:00–16:00 UTC Apr 7 13:00–14:00 UTC Apr 8 10:00–11:00 UTC Apr 9 10:00–11:00 UTC Apr 10 12:00–13:00 UTC Apr 12 12:00–14:00 UTC Apr 13 11:00–13:00 UTC Apr 14 00:00–01:00, 12:00–14:00 UTC Apr 15 12:00–13:00 UTC Apr 16 12:00–14:00 UTC Apr 17 12:00–15:00 UTC Apr 18 14:00–17:00 UTC Apr 19 01:00–04:00, 12:00–15:00 UTC Apr 20 12:00–14:00 UTC Apr 21 12:00–14:00 UTC Apr 23 12:00–15:00 UTC Apr 25 01:00–02:00, 13:00–16:00 UTC Apr 26 06:00–08:00, 15:00–19:00 UTC Apr 27 13:00–15:00 UTC Apr 29 12:00–14:00 UTC Apr 30 10:00–12:00 UTC
May
May 1 12:00–13:00 UTC May 2 13:00–14:00 UTC May 3 14:00–19:00 UTC May 4 12:00–13:00 UTC May 5 13:00–14:00 UTC May 6 13:00–15:00 UTC May 7 04:00–08:00, 13:00–15:00 UTC May 8 12:00–15:00 UTC May 9 15:00–17:00 UTC May 10 12:00–14:00 UTC May 11 13:00–14:00 UTC May 12 14:00–15:00 UTC May 13 05:00–07:00, 13:00–14:00 UTC May 14 13:00–16:00 UTC May 15 12:00–13:00 UTC May 16 09:00–12:00 UTC May 17 03:00–05:00, 13:00–15:00 UTC May 18 03:00–05:00, 12:00–13:00 UTC May 19 04:00–05:00, 12:00–15:00, 23:00–24:00 UTC May 20 12:00–13:00 UTC May 21 10:00–12:00 UTC May 22 13:00–15:00, 23:00–24:00 UTC May 23 12:00–14:00, 23:00–24:00 UTC May 24 14:00–16:00 UTC May 25 01:00–02:00 UTC May 26 12:00–15:00, 23:00–24:00 UTC May 27 01:00–02:00, 22:00–23:00 UTC May 28 00:00–01:00, 22:00–24:00 UTC May 29 00:00–01:00, 12:00–15:00 UTC May 30 13:00–17:00 UTC May 31 14:00–16:00 UTC
June
Jun 1 01:00–04:00, 14:00–15:00 UTC Jun 2 11:00–13:00 UTC Jun 3 15:00–18:00 UTC Jun 5 14:00–17:00 UTC Jun 6 00:00–01:00, 16:00–18:00 UTC Jun 7 13:00–16:00 UTC Jun 8 13:00–15:00 UTC Jun 9 15:00–17:00 UTC Jun 10 22:00–24:00 UTC Jun 11 13:00–16:00 UTC Jun 12 15:00–17:00 UTC Jun 13 14:00–17:00 UTC Jun 15 11:00–14:00 UTC Jun 18 15:00–17:00 UTC Jun 19 12:00–14:00 UTC Jun 21 12:00–16:00 UTC Jun 22 13:00–15:00 UTC Jun 24 14:00–15:00 UTC Jun 25 13:00–16:00 UTC Jun 26 03:00–05:00, 12:00–15:00 UTC Jun 27 01:00–02:00, 15:00–17:00 UTC Jun 28 02:00–05:00, 12:00–16:00 UTC Jun 29 13:00–15:00 UTC Jun 30 01:00–04:00, 12:00–14:00 UTC
July
Jul 1 14:00–16:00 UTC Jul 2 15:00–16:00 UTC Jul 3 06:00–08:00, 14:00–17:00 UTC Jul 4 12:00–14:00 UTC Jul 5 07:00–08:00, 14:00–17:00 UTC Jul 6 14:00–15:00 UTC Jul 7 13:00–15:00 UTC Jul 8 08:00–10:00, 14:00–15:00 UTC Jul 9 00:00–01:00, 14:00–18:00 UTC Jul 10 13:00–16:00 UTC Jul 11 11:00–13:00 UTC Jul 12 06:00–07:00, 18:00–19:00 UTC Jul 13 12:00–14:00 UTC Jul 14 22:00–24:00 UTC Jul 15 12:00–14:00 UTC Jul 18 14:00–16:00 UTC Jul 21 14:00–17:00 UTC Jul 22 13:00–16:00 UTC Jul 23 02:00–04:00 UTC Jul 24 15:00–17:00 UTC Jul 25 20:00–21:00 UTC Jul 26 04:00–05:00, 12:00–16:00, 19:00–20:00 UTC Jul 27 12:00–13:00 UTC