Most often in June, 24 days.
Highest follower count seen that year: 37,317.
January
Jan 6 12:00–19:00 UTC Jan 7 16:00–22:00 UTC Jan 8 12:00–20:00 UTC Jan 9 13:00–18:00 UTC Jan 13 12:00–20:00 UTC Jan 14 14:00–19:00 UTC Jan 15 14:00–22:00 UTC Jan 16 14:00–20:00 UTC Jan 17 13:00–19:00 UTC Jan 21 12:00–21:00 UTC Jan 22 15:00–22:00 UTC Jan 23 14:00–20:00 UTC Jan 24 14:00–22:00 UTC Jan 26 14:00–18:00 UTC Jan 27 13:00–22:00 UTC Jan 28 12:00–16:00, 18:00–22:00 UTC Jan 29 14:00–17:00 UTC Jan 31 14:00–19:00 UTC
February
Feb 3 13:00–20:00 UTC Feb 4 15:00–19:00 UTC Feb 6 13:00–19:00 UTC Feb 7 14:00–16:00, 18:00–21:00 UTC Feb 9 14:00–20:00 UTC Feb 10 13:00–21:00 UTC Feb 11 14:00–22:00 UTC Feb 13 14:00–20:00 UTC Feb 14 13:00–22:00 UTC Feb 16 13:00–21:00 UTC Feb 18 17:00–22:00 UTC Feb 20 14:00–18:00 UTC Feb 21 14:00–20:00 UTC Feb 24 18:00–24:00 UTC Feb 26 14:00–21:00 UTC Feb 27 15:00–21:00 UTC Feb 28 13:00–20:00 UTC
March
Mar 2 13:00–22:00 UTC Mar 4 13:00–21:00 UTC Mar 5 13:00–21:00 UTC Mar 6 13:00–21:00 UTC Mar 7 13:00–18:00 UTC Mar 9 14:00–16:00 UTC Mar 12 13:00–20:00 UTC Mar 14 13:00–21:00 UTC Mar 16 13:00–19:00 UTC Mar 18 13:00–18:00, 20:00–21:00 UTC Mar 19 13:00–20:00 UTC Mar 20 14:00–21:00 UTC Mar 23 13:00–19:00 UTC Mar 24 13:00–20:00 UTC Mar 25 14:00–21:00 UTC Mar 26 14:00–17:00, 19:00–21:00 UTC Mar 28 13:00–22:00 UTC Mar 30 14:00–22:00 UTC Mar 31 14:00–20:00 UTC
April
Apr 1 13:00–18:00 UTC Apr 2 14:00–19:00 UTC Apr 4 14:00–22:00 UTC Apr 6 14:00–20:00 UTC Apr 7 14:00–22:00 UTC Apr 9 13:00–19:00 UTC Apr 10 13:00–21:00 UTC Apr 11 13:00–21:00 UTC Apr 13 14:00–22:00 UTC Apr 14 19:00–24:00 UTC Apr 16 14:00–22:00 UTC Apr 17 14:00–19:00 UTC Apr 18 14:00–20:00 UTC Apr 20 14:00–20:00 UTC Apr 22 14:00–20:00 UTC Apr 23 13:00–21:00 UTC Apr 24 14:00–22:00 UTC Apr 25 13:00–22:00 UTC Apr 28 14:00–22:00 UTC Apr 29 14:00–21:00 UTC Apr 30 14:00–21:00 UTC
May
May 1 20:00–24:00 UTC May 2 14:00–20:00 UTC May 6 14:00–21:00 UTC May 7 14:00–20:00 UTC May 8 14:00–15:00, 18:00–21:00 UTC May 9 14:00–18:00 UTC May 13 14:00–21:00 UTC May 14 14:00–21:00 UTC May 16 14:00–20:00 UTC May 18 14:00–20:00 UTC May 20 14:00–21:00 UTC May 21 16:00–22:00 UTC May 22 14:00–21:00 UTC May 23 14:00–20:00 UTC May 25 14:00–20:00 UTC May 27 12:00–18:00 UTC May 28 13:00–19:00 UTC
June
Jun 2 13:00–19:00 UTC Jun 4 15:00–22:00 UTC Jun 5 15:00–17:00 UTC Jun 6 16:00–19:00 UTC Jun 7 15:00–21:00 UTC Jun 8 02:00–04:00 UTC Jun 9 16:00–24:00 UTC Jun 10 00:00–02:00 UTC Jun 11 15:00–21:00, 23:00–24:00 UTC Jun 12 00:00–02:00, 17:00–18:00, 23:00–24:00 UTC Jun 13 00:00–02:00 UTC Jun 15 15:00–18:00, 21:00–24:00 UTC Jun 16 00:00–01:00, 22:00–24:00 UTC Jun 17 00:00–02:00, 19:00–24:00 UTC Jun 18 00:00–04:00, 17:00–24:00 UTC Jun 19 00:00–02:00, 16:00–24:00 UTC Jun 20 00:00–02:00, 14:00–17:00, 21:00–24:00 UTC Jun 21 00:00–05:00, 23:00–24:00 UTC Jun 22 00:00–02:00 UTC Jun 25 14:00–20:00, 23:00–24:00 UTC Jun 26 00:00–01:00, 15:00–20:00 UTC Jun 27 00:00–04:00, 16:00–19:00, 23:00–24:00 UTC Jun 28 00:00–04:00 UTC Jun 30 14:00–17:00, 21:00–24:00 UTC
July
Jul 1 00:00–01:00, 15:00–18:00, 21:00–24:00 UTC Jul 2 00:00–01:00, 16:00–19:00, 21:00–24:00 UTC Jul 3 00:00–01:00, 17:00–21:00, 23:00–24:00 UTC Jul 4 00:00–02:00, 14:00–20:00 UTC Jul 6 14:00–15:00, 18:00–20:00, 23:00–24:00 UTC Jul 8 18:00–20:00, 22:00–24:00 UTC Jul 9 00:00–04:00, 19:00–21:00 UTC Jul 10 01:00–03:00, 15:00–20:00, 23:00–24:00 UTC Jul 11 00:00–01:00, 15:00–21:00 UTC Jul 12 00:00–02:00 UTC Jul 16 14:00–17:00, 20:00–24:00 UTC Jul 17 00:00–02:00, 23:00–24:00 UTC Jul 18 00:00–01:00, 15:00–23:00 UTC Jul 21 14:00–17:00 UTC Jul 25 19:00–24:00 UTC Jul 26 16:00–18:00, 19:00–23:00 UTC Jul 27 14:00–15:00, 17:00–18:00, 20:00–21:00 UTC