Most often in May, 27 days.
Highest follower count seen that year: 31,209.
January
Jan 4 18:00–24:00 UTC Jan 5 00:00–01:00, 17:00–22:00 UTC Jan 6 16:00–22:00 UTC Jan 8 17:00–21:00 UTC Jan 9 18:00–24:00 UTC Jan 10 00:00–01:00, 18:00–24:00 UTC Jan 11 00:00–01:00, 19:00–23:00 UTC Jan 12 01:00–04:00 UTC Jan 13 17:00–24:00 UTC Jan 15 17:00–22:00 UTC Jan 17 23:00–24:00 UTC Jan 18 18:00–23:00 UTC Jan 19 00:00–02:00, 18:00–21:00 UTC Jan 20 18:00–24:00 UTC Jan 21 00:00–01:00, 17:00–23:00 UTC Jan 22 17:00–23:00 UTC Jan 23 16:00–22:00 UTC Jan 25 22:00–24:00 UTC Jan 26 00:00–01:00, 17:00–22:00 UTC Jan 27 16:00–22:00 UTC Jan 28 17:00–20:00 UTC Jan 29 17:00–22:00 UTC Jan 31 19:00–24:00 UTC
February
Feb 1 00:00–01:00 UTC Feb 3 17:00–23:00 UTC Feb 5 21:00–24:00 UTC Feb 8 23:00–24:00 UTC Feb 9 00:00–02:00, 17:00–22:00 UTC Feb 10 18:00–24:00 UTC Feb 16 16:00–22:00 UTC Feb 24 17:00–22:00 UTC Feb 25 20:00–22:00 UTC Feb 26 18:00–20:00 UTC Feb 28 23:00–24:00 UTC
March
Mar 1 00:00–05:00 UTC Mar 2 20:00–23:00 UTC Mar 4 18:00–24:00 UTC Mar 5 00:00–01:00 UTC Mar 6 23:00–24:00 UTC Mar 7 20:00–24:00 UTC Mar 8 00:00–02:00, 16:00–20:00, 22:00–24:00 UTC Mar 9 00:00–01:00, 20:00–24:00 UTC Mar 10 00:00–01:00, 20:00–24:00 UTC Mar 11 18:00–24:00 UTC Mar 12 00:00–01:00, 19:00–23:00 UTC Mar 15 19:00–24:00 UTC Mar 16 00:00–01:00, 18:00–24:00 UTC Mar 17 18:00–22:00 UTC Mar 19 18:00–22:00 UTC Mar 21 21:00–24:00 UTC Mar 22 00:00–02:00, 17:00–23:00 UTC Mar 24 18:00–24:00 UTC Mar 25 18:00–22:00 UTC Mar 26 20:00–24:00 UTC Mar 28 22:00–24:00 UTC Mar 29 00:00–01:00, 21:00–24:00 UTC Mar 30 00:00–02:00, 20:00–24:00 UTC Mar 31 00:00–01:00, 20:00–23:00 UTC
April
Apr 1 19:00–23:00 UTC Apr 4 20:00–23:00 UTC Apr 5 22:00–24:00 UTC Apr 6 00:00–01:00 UTC Apr 7 19:00–24:00 UTC Apr 12 23:00–24:00 UTC Apr 13 00:00–01:00 UTC Apr 14 20:00–24:00 UTC Apr 15 19:00–23:00 UTC Apr 16 19:00–23:00 UTC Apr 19 00:00–03:00, 18:00–24:00 UTC Apr 20 00:00–02:00, 13:00–14:00, 20:00–24:00 UTC Apr 21 19:00–23:00 UTC Apr 23 21:00–24:00 UTC Apr 25 18:00–23:00 UTC Apr 26 15:00–22:00 UTC Apr 27 19:00–24:00 UTC Apr 28 18:00–23:00 UTC Apr 29 18:00–23:00 UTC Apr 30 18:00–23:00 UTC
May
May 2 20:00–24:00 UTC May 3 00:00–02:00, 15:00–24:00 UTC May 4 19:00–24:00 UTC May 5 17:00–23:00 UTC May 7 18:00–24:00 UTC May 8 00:00–01:00 UTC May 9 19:00–24:00 UTC May 10 00:00–02:00, 14:00–22:00 UTC May 11 21:00–24:00 UTC May 12 00:00–02:00 UTC May 13 18:00–23:00 UTC May 14 18:00–23:00 UTC May 16 18:00–22:00 UTC May 17 18:00–24:00 UTC May 18 00:00–01:00, 22:00–24:00 UTC May 19 17:00–24:00 UTC May 20 18:00–23:00 UTC May 21 20:00–23:00 UTC May 22 21:00–24:00 UTC May 23 00:00–01:00, 19:00–23:00 UTC May 24 12:00–17:00 UTC May 25 18:00–24:00 UTC May 26 00:00–01:00, 21:00–24:00 UTC May 27 00:00–01:00, 18:00–23:00 UTC May 28 18:00–24:00 UTC May 30 19:00–24:00 UTC May 31 00:00–02:00, 13:00–24:00 UTC
June
Jun 1 00:00–01:00, 22:00–24:00 UTC Jun 2 00:00–04:00, 19:00–22:00 UTC Jun 3 00:00–02:00, 19:00–24:00 UTC Jun 5 23:00–24:00 UTC Jun 6 00:00–02:00, 18:00–24:00 UTC Jun 7 00:00–04:00, 13:00–24:00 UTC Jun 8 00:00–02:00, 21:00–24:00 UTC Jun 9 00:00–03:00, 18:00–24:00 UTC Jun 10 00:00–02:00, 23:00–24:00 UTC Jun 11 00:00–02:00, 18:00–24:00 UTC Jun 13 12:00–19:00 UTC Jun 15 21:00–24:00 UTC Jun 16 00:00–02:00, 20:00–24:00 UTC Jun 17 19:00–24:00 UTC Jun 18 00:00–01:00 UTC Jun 19 19:00–24:00 UTC Jun 20 00:00–02:00, 17:00–24:00 UTC Jun 21 12:00–21:00 UTC Jun 22 18:00–24:00 UTC Jun 23 00:00–01:00, 17:00–24:00 UTC Jun 25 19:00–24:00 UTC Jun 26 00:00–01:00 UTC Jun 28 12:00–16:00, 20:00–24:00 UTC Jun 29 00:00–01:00 UTC
July
Jul 6 20:00–24:00 UTC Jul 7 00:00–02:00, 18:00–21:00 UTC Jul 9 20:00–24:00 UTC Jul 10 01:00–04:00, 18:00–24:00 UTC Jul 11 00:00–01:00 UTC Jul 12 13:00–24:00 UTC Jul 13 00:00–01:00, 19:00–23:00 UTC Jul 19 22:00–24:00 UTC Jul 20 00:00–01:00 UTC