Most often in May, 29 days.
Highest follower count seen that year: 81,034.
January
Jan 2 16:00–17:00 UTC Jan 5 02:00–04:00 UTC Jan 6 11:00–13:00 UTC Jan 7 02:00–05:00 UTC Jan 9 14:00–17:00 UTC Jan 10 15:00–18:00, 20:00–22:00 UTC Jan 11 14:00–17:00, 23:00–24:00 UTC Jan 12 00:00–01:00, 23:00–24:00 UTC Jan 13 00:00–02:00, 14:00–17:00, 20:00–23:00 UTC Jan 14 18:00–21:00 UTC Jan 15 15:00–18:00 UTC Jan 16 17:00–20:00, 23:00–24:00 UTC Jan 17 15:00–18:00 UTC Jan 18 16:00–19:00 UTC Jan 19 00:00–01:00, 15:00–17:00 UTC Jan 20 15:00–18:00, 22:00–24:00 UTC Jan 21 15:00–18:00 UTC Jan 22 14:00–17:00 UTC Jan 23 16:00–19:00 UTC Jan 25 00:00–02:00, 16:00–18:00 UTC Jan 30 10:00–12:00, 21:00–22:00 UTC
February
Feb 5 00:00–01:00 UTC Feb 10 14:00–17:00 UTC Feb 11 15:00–21:00 UTC Feb 12 14:00–18:00 UTC Feb 14 20:00–22:00 UTC Feb 15 21:00–24:00 UTC Feb 16 15:00–18:00 UTC Feb 17 15:00–18:00 UTC Feb 18 19:00–21:00 UTC Feb 21 15:00–18:00 UTC Feb 22 15:00–17:00 UTC Feb 27 23:00–24:00 UTC Feb 28 00:00–02:00, 14:00–18:00 UTC
March
Mar 2 00:00–03:00, 14:00–18:00 UTC Mar 3 16:00–18:00 UTC Mar 4 23:00–24:00 UTC Mar 5 00:00–01:00, 15:00–17:00 UTC Mar 6 23:00–24:00 UTC Mar 7 00:00–01:00, 15:00–18:00 UTC Mar 9 21:00–22:00 UTC Mar 10 16:00–18:00, 23:00–24:00 UTC Mar 11 00:00–01:00, 15:00–20:00 UTC Mar 12 16:00–18:00 UTC Mar 13 21:00–23:00 UTC Mar 14 01:00–03:00, 15:00–18:00, 23:00–24:00 UTC Mar 15 00:00–01:00, 16:00–17:00 UTC Mar 18 23:00–24:00 UTC Mar 19 15:00–18:00 UTC Mar 20 00:00–02:00, 14:00–15:00 UTC Mar 21 15:00–18:00 UTC Mar 22 15:00–20:00 UTC Mar 23 16:00–19:00, 21:00–23:00 UTC Mar 24 22:00–24:00 UTC
April
Apr 1 21:00–24:00 UTC Apr 2 00:00–02:00, 15:00–18:00 UTC Apr 4 14:00–16:00 UTC Apr 5 14:00–16:00 UTC Apr 7 00:00–03:00, 15:00–17:00 UTC Apr 10 16:00–18:00 UTC Apr 11 15:00–17:00, 22:00–24:00 UTC Apr 12 15:00–16:00 UTC Apr 13 15:00–17:00, 20:00–21:00 UTC Apr 14 12:00–14:00 UTC Apr 17 15:00–16:00 UTC Apr 18 15:00–17:00, 21:00–23:00 UTC Apr 19 13:00–15:00 UTC Apr 20 14:00–15:00 UTC Apr 22 00:00–03:00, 14:00–15:00 UTC Apr 23 17:00–19:00 UTC Apr 24 15:00–18:00 UTC Apr 25 14:00–16:00, 22:00–24:00 UTC Apr 26 00:00–01:00, 14:00–17:00 UTC Apr 27 14:00–17:00 UTC Apr 28 15:00–17:00, 21:00–22:00 UTC Apr 29 14:00–19:00 UTC Apr 30 00:00–03:00, 13:00–17:00 UTC
May
May 1 16:00–18:00 UTC May 2 14:00–16:00, 23:00–24:00 UTC May 3 15:00–17:00, 21:00–23:00 UTC May 4 14:00–19:00 UTC May 5 16:00–17:00 UTC May 6 14:00–18:00 UTC May 7 15:00–17:00 UTC May 8 00:00–02:00, 16:00–18:00, 23:00–24:00 UTC May 9 16:00–17:00 UTC May 10 15:00–18:00 UTC May 11 15:00–18:00 UTC May 12 14:00–15:00 UTC May 13 15:00–18:00, 20:00–23:00 UTC May 14 14:00–18:00 UTC May 15 15:00–18:00, 22:00–24:00 UTC May 16 00:00–01:00, 15:00–17:00, 22:00–23:00 UTC May 17 14:00–19:00 UTC May 18 14:00–18:00, 21:00–24:00 UTC May 19 14:00–17:00, 22:00–24:00 UTC May 20 00:00–01:00, 14:00–17:00 UTC May 21 15:00–18:00, 21:00–24:00 UTC May 22 15:00–17:00 UTC May 23 14:00–17:00 UTC May 24 14:00–16:00 UTC May 25 14:00–18:00, 20:00–24:00 UTC May 26 15:00–17:00, 21:00–24:00 UTC May 27 15:00–18:00 UTC May 28 14:00–18:00, 21:00–24:00 UTC May 29 00:00–02:00, 15:00–19:00 UTC
June
Jun 1 14:00–19:00 UTC Jun 2 14:00–19:00, 21:00–24:00 UTC Jun 3 13:00–17:00 UTC Jun 4 14:00–18:00 UTC Jun 5 14:00–18:00 UTC Jun 6 00:00–01:00, 14:00–17:00, 22:00–24:00 UTC Jun 7 00:00–01:00, 16:00–19:00 UTC Jun 9 20:00–24:00 UTC Jun 10 15:00–18:00 UTC Jun 11 15:00–18:00 UTC Jun 12 15:00–18:00 UTC Jun 13 15:00–18:00 UTC Jun 15 15:00–18:00 UTC Jun 22 15:00–18:00 UTC Jun 23 14:00–17:00, 21:00–24:00 UTC Jun 24 15:00–18:00, 20:00–23:00 UTC Jun 25 14:00–18:00 UTC Jun 26 14:00–18:00 UTC Jun 27 21:00–24:00 UTC Jun 28 15:00–18:00 UTC Jun 29 14:00–18:00 UTC Jun 30 14:00–17:00 UTC
July
Jul 1 15:00–18:00 UTC Jul 3 14:00–17:00 UTC Jul 4 15:00–18:00 UTC Jul 5 16:00–17:00, 20:00–22:00 UTC Jul 6 14:00–18:00 UTC Jul 7 14:00–17:00 UTC Jul 8 15:00–20:00 UTC Jul 10 00:00–01:00, 15:00–18:00, 23:00–24:00 UTC Jul 11 00:00–01:00, 14:00–18:00 UTC Jul 12 15:00–17:00, 21:00–23:00 UTC Jul 13 14:00–18:00, 22:00–24:00 UTC Jul 14 14:00–18:00, 23:00–24:00 UTC Jul 15 14:00–17:00 UTC Jul 16 15:00–18:00 UTC Jul 17 15:00–17:00 UTC Jul 18 15:00–17:00 UTC Jul 19 00:00–02:00, 13:00–16:00 UTC Jul 22 16:00–19:00 UTC Jul 23 16:00–17:00, 21:00–22:00 UTC Jul 26 15:00–19:00 UTC Jul 27 14:00–15:00, 16:00–18:00 UTC