Most often in May, 30 days.
Highest follower count seen that year: 1,387.
February
Feb 23 20:00–24:00 UTC Feb 24 00:00–03:00 UTC Feb 25 03:00–08:00 UTC Feb 26 20:00–24:00 UTC Feb 27 00:00–03:00, 19:00–24:00 UTC Feb 28 00:00–04:00, 19:00–24:00 UTC
March
Mar 1 00:00–05:00, 19:00–24:00 UTC Mar 2 00:00–02:00, 21:00–24:00 UTC Mar 3 00:00–02:00, 20:00–24:00 UTC Mar 4 00:00–02:00 UTC Mar 5 12:00–18:00 UTC Mar 6 08:00–09:00, 17:00–21:00 UTC Mar 7 20:00–24:00 UTC Mar 8 00:00–03:00, 20:00–24:00 UTC Mar 9 00:00–01:00, 21:00–24:00 UTC Mar 10 00:00–02:00, 20:00–24:00 UTC Mar 11 00:00–02:00 UTC Mar 12 20:00–24:00 UTC Mar 13 00:00–03:00, 20:00–24:00 UTC Mar 14 00:00–03:00, 19:00–24:00 UTC Mar 15 00:00–03:00, 20:00–24:00 UTC Mar 16 00:00–02:00, 20:00–24:00 UTC Mar 17 00:00–03:00, 21:00–22:00 UTC Mar 19 19:00–24:00 UTC Mar 20 00:00–02:00 UTC Mar 21 21:00–24:00 UTC Mar 22 00:00–02:00, 19:00–24:00 UTC Mar 23 00:00–03:00, 21:00–24:00 UTC Mar 24 00:00–02:00, 19:00–24:00 UTC Mar 25 00:00–02:00 UTC Mar 27 18:00–24:00 UTC Mar 28 00:00–03:00, 20:00–24:00 UTC Mar 29 00:00–03:00 UTC Mar 30 20:00–24:00 UTC Mar 31 00:00–02:00, 20:00–22:00 UTC
April
Apr 1 00:00–02:00 UTC Apr 2 00:00–02:00, 17:00–24:00 UTC Apr 3 00:00–02:00 UTC Apr 4 20:00–24:00 UTC Apr 5 00:00–03:00, 20:00–24:00 UTC Apr 6 00:00–03:00, 20:00–24:00 UTC Apr 7 00:00–01:00, 22:00–24:00 UTC Apr 8 00:00–02:00 UTC Apr 9 00:00–02:00, 23:00–24:00 UTC Apr 10 00:00–02:00 UTC Apr 11 20:00–24:00 UTC Apr 12 00:00–02:00, 20:00–24:00 UTC Apr 13 00:00–02:00, 21:00–24:00 UTC Apr 14 00:00–01:00, 20:00–24:00 UTC Apr 15 00:00–08:00, 20:00–24:00 UTC Apr 16 00:00–01:00 UTC Apr 17 20:00–24:00 UTC Apr 18 00:00–02:00 UTC Apr 20 20:00–24:00 UTC Apr 21 00:00–02:00, 20:00–24:00 UTC Apr 22 00:00–02:00, 20:00–24:00 UTC Apr 23 00:00–08:00, 20:00–24:00 UTC Apr 24 00:00–01:00, 20:00–24:00 UTC Apr 25 00:00–06:00 UTC Apr 27 20:00–24:00 UTC Apr 28 00:00–03:00, 20:00–23:00 UTC Apr 29 00:00–01:00, 20:00–24:00 UTC Apr 30 00:00–08:00, 20:00–24:00 UTC
May
May 1 00:00–02:00, 20:00–24:00 UTC May 2 00:00–02:00, 19:00–20:00 UTC May 3 00:00–03:00 UTC May 4 20:00–24:00 UTC May 5 00:00–02:00, 20:00–24:00 UTC May 6 00:00–03:00 UTC May 7 20:00–21:00 UTC May 8 18:00–24:00 UTC May 9 00:00–01:00, 20:00–24:00 UTC May 10 00:00–03:00, 20:00–24:00 UTC May 11 00:00–02:00, 21:00–24:00 UTC May 12 00:00–05:00, 21:00–24:00 UTC May 13 00:00–03:00 UTC May 14 20:00–24:00 UTC May 15 16:00–22:00 UTC May 17 19:00–24:00 UTC May 18 00:00–03:00, 20:00–24:00 UTC May 19 00:00–03:00, 20:00–24:00 UTC May 20 00:00–02:00, 20:00–22:00 UTC May 21 01:00–03:00, 20:00–21:00 UTC May 22 01:00–02:00, 21:00–22:00 UTC May 23 01:00–03:00, 21:00–24:00 UTC May 24 00:00–02:00 UTC May 25 11:00–19:00 UTC May 26 21:00–24:00 UTC May 27 00:00–02:00 UTC May 28 20:00–24:00 UTC May 29 00:00–01:00, 20:00–24:00 UTC May 30 00:00–03:00, 20:00–24:00 UTC May 31 00:00–02:00 UTC
June
Jun 1 20:00–24:00 UTC Jun 2 00:00–01:00 UTC Jun 9 20:00–24:00 UTC Jun 10 00:00–03:00 UTC Jun 11 20:00–24:00 UTC Jun 12 00:00–03:00, 21:00–24:00 UTC Jun 13 00:00–03:00 UTC Jun 15 01:00–07:00, 13:00–24:00 UTC Jun 16 00:00–01:00, 19:00–24:00 UTC Jun 17 00:00–02:00 UTC Jun 18 20:00–24:00 UTC Jun 19 00:00–02:00, 20:00–23:00 UTC Jun 20 00:00–02:00, 19:00–24:00 UTC Jun 22 19:00–24:00 UTC Jun 23 00:00–01:00, 23:00–24:00 UTC Jun 24 00:00–04:00 UTC Jun 25 20:00–24:00 UTC Jun 26 00:00–02:00, 19:00–24:00 UTC Jun 27 00:00–01:00 UTC Jun 29 20:00–24:00 UTC Jun 30 00:00–02:00, 20:00–24:00 UTC
July
Jul 2 20:00–24:00 UTC Jul 3 00:00–02:00, 20:00–24:00 UTC Jul 4 00:00–02:00, 20:00–24:00 UTC Jul 5 00:00–04:00, 20:00–24:00 UTC Jul 6 00:00–03:00, 20:00–24:00 UTC Jul 7 00:00–02:00 UTC Jul 8 00:00–03:00, 15:00–16:00, 18:00–19:00 UTC Jul 9 20:00–24:00 UTC Jul 10 00:00–03:00, 22:00–23:00 UTC Jul 11 00:00–08:00, 20:00–24:00 UTC Jul 12 01:00–02:00, 20:00–24:00 UTC Jul 13 00:00–02:00, 20:00–24:00 UTC Jul 14 00:00–03:00, 20:00–24:00 UTC Jul 15 00:00–03:00 UTC Jul 16 20:00–24:00 UTC Jul 17 00:00–03:00, 22:00–24:00 UTC Jul 18 00:00–05:00, 20:00–24:00 UTC Jul 19 00:00–02:00, 20:00–24:00 UTC Jul 20 00:00–05:00, 22:00–23:00 UTC Jul 21 00:00–03:00, 20:00–24:00 UTC Jul 22 00:00–08:00 UTC Jul 24 01:00–04:00, 22:00–24:00 UTC Jul 25 00:00–01:00 UTC Jul 26 23:00–24:00 UTC Jul 27 00:00–02:00 UTC