Most often in July, 27 days.
Highest follower count seen that year: 24,388.
January
Jan 1 03:00–05:00, 20:00–22:00 UTC Jan 3 20:00–22:00 UTC Jan 4 01:00–02:00, 22:00–23:00 UTC Jan 6 20:00–23:00 UTC Jan 7 19:00–23:00 UTC Jan 8 19:00–22:00 UTC Jan 9 21:00–23:00 UTC Jan 10 15:00–21:00 UTC Jan 11 21:00–24:00 UTC Jan 12 19:00–24:00 UTC Jan 13 17:00–19:00 UTC Jan 14 18:00–22:00 UTC Jan 15 18:00–21:00 UTC Jan 17 16:00–18:00, 21:00–23:00 UTC Jan 19 19:00–23:00 UTC Jan 20 18:00–21:00 UTC Jan 22 00:00–02:00, 20:00–23:00 UTC Jan 23 18:00–22:00 UTC Jan 24 14:00–22:00 UTC Jan 25 18:00–20:00, 22:00–23:00 UTC Jan 26 19:00–23:00 UTC Jan 27 18:00–20:00 UTC Jan 28 20:00–24:00 UTC Jan 29 20:00–24:00 UTC Jan 30 00:00–02:00, 20:00–24:00 UTC Jan 31 00:00–01:00, 15:00–20:00 UTC
February
Feb 1 18:00–24:00 UTC Feb 3 19:00–23:00 UTC Feb 4 18:00–21:00 UTC Feb 5 19:00–23:00 UTC Feb 6 19:00–23:00 UTC Feb 7 14:00–17:00, 21:00–23:00 UTC Feb 9 20:00–21:00 UTC Feb 10 20:00–24:00 UTC Feb 11 00:00–02:00, 20:00–23:00 UTC Feb 12 20:00–21:00 UTC Feb 13 01:00–03:00 UTC Feb 14 03:00–04:00, 14:00–16:00, 18:00–19:00 UTC Feb 16 19:00–20:00 UTC Feb 18 20:00–23:00 UTC Feb 19 19:00–23:00 UTC Feb 20 19:00–20:00 UTC Feb 21 03:00–04:00, 14:00–17:00 UTC Feb 22 01:00–04:00, 14:00–17:00, 20:00–22:00 UTC Feb 23 01:00–03:00, 19:00–22:00 UTC Feb 24 22:00–24:00 UTC Feb 25 00:00–02:00, 20:00–23:00 UTC Feb 26 19:00–20:00 UTC Feb 28 18:00–23:00 UTC
March
Mar 1 15:00–17:00, 19:00–22:00 UTC Mar 2 19:00–24:00 UTC Mar 3 19:00–23:00 UTC Mar 4 20:00–24:00 UTC Mar 5 00:00–01:00, 03:00–05:00, 19:00–22:00 UTC Mar 6 19:00–21:00 UTC Mar 7 01:00–04:00, 15:00–18:00 UTC Mar 8 14:00–16:00 UTC Mar 9 20:00–23:00 UTC Mar 10 01:00–03:00, 18:00–22:00 UTC Mar 11 18:00–22:00 UTC Mar 12 20:00–24:00 UTC Mar 13 14:00–15:00, 18:00–19:00 UTC Mar 16 18:00–21:00 UTC Mar 17 01:00–03:00, 20:00–23:00 UTC Mar 18 00:00–04:00, 19:00–22:00 UTC Mar 19 00:00–03:00, 18:00–23:00 UTC Mar 21 00:00–02:00, 14:00–16:00, 19:00–20:00 UTC Mar 22 02:00–04:00, 15:00–18:00, 20:00–24:00 UTC Mar 23 19:00–21:00 UTC Mar 24 19:00–24:00 UTC Mar 25 00:00–03:00, 19:00–24:00 UTC Mar 26 00:00–02:00, 19:00–24:00 UTC Mar 27 00:00–01:00, 20:00–24:00 UTC Mar 28 15:00–17:00 UTC Mar 31 20:00–24:00 UTC
April
Apr 1 19:00–23:00 UTC Apr 2 19:00–23:00 UTC Apr 4 14:00–18:00, 21:00–23:00 UTC Apr 5 15:00–17:00, 20:00–24:00 UTC Apr 6 00:00–01:00 UTC Apr 7 00:00–01:00 UTC Apr 13 20:00–22:00 UTC Apr 14 21:00–24:00 UTC Apr 15 18:00–22:00 UTC Apr 16 20:00–24:00 UTC Apr 17 00:00–01:00, 20:00–24:00 UTC Apr 18 14:00–17:00, 20:00–23:00 UTC Apr 19 20:00–24:00 UTC Apr 20 19:00–22:00 UTC Apr 21 22:00–24:00 UTC Apr 22 00:00–02:00, 19:00–24:00 UTC Apr 23 20:00–22:00 UTC Apr 24 19:00–24:00 UTC Apr 25 00:00–01:00, 15:00–18:00, 21:00–24:00 UTC Apr 28 20:00–24:00 UTC Apr 29 21:00–24:00 UTC Apr 30 00:00–01:00, 19:00–24:00 UTC
May
May 1 00:00–01:00, 19:00–24:00 UTC May 2 15:00–17:00, 20:00–22:00 UTC May 3 18:00–22:00 UTC May 6 22:00–24:00 UTC May 7 00:00–02:00, 19:00–22:00 UTC May 8 19:00–24:00 UTC May 9 00:00–01:00, 14:00–20:00 UTC May 10 18:00–21:00, 23:00–24:00 UTC May 11 00:00–01:00, 20:00–24:00 UTC May 13 20:00–22:00 UTC May 14 18:00–22:00 UTC May 15 00:00–01:00, 05:00–07:00, 19:00–22:00 UTC May 16 00:00–01:00, 14:00–17:00, 19:00–24:00 UTC May 17 00:00–01:00, 14:00–16:00, 21:00–23:00 UTC May 20 21:00–22:00 UTC May 21 21:00–24:00 UTC May 23 19:00–23:00 UTC May 24 14:00–17:00 UTC May 25 19:00–23:00 UTC May 26 00:00–01:00, 18:00–21:00 UTC May 27 01:00–03:00, 20:00–23:00 UTC May 28 20:00–24:00 UTC May 29 17:00–19:00, 23:00–24:00 UTC May 30 00:00–01:00, 15:00–17:00, 21:00–24:00 UTC May 31 00:00–01:00, 13:00–17:00, 19:00–24:00 UTC
June
Jun 2 19:00–23:00 UTC Jun 3 19:00–23:00 UTC Jun 4 20:00–24:00 UTC Jun 5 00:00–01:00, 19:00–21:00 UTC Jun 6 01:00–03:00, 14:00–17:00, 19:00–22:00 UTC Jun 7 20:00–24:00 UTC Jun 8 20:00–22:00 UTC Jun 9 20:00–23:00 UTC Jun 10 20:00–24:00 UTC Jun 13 16:00–18:00 UTC Jun 15 19:00–24:00 UTC Jun 16 00:00–01:00, 19:00–23:00 UTC Jun 17 00:00–02:00, 19:00–23:00 UTC Jun 18 19:00–24:00 UTC Jun 19 19:00–24:00 UTC Jun 20 13:00–17:00, 19:00–21:00 UTC Jun 21 18:00–23:00 UTC Jun 22 19:00–24:00 UTC Jun 23 19:00–22:00 UTC Jun 24 19:00–20:00 UTC Jun 26 21:00–24:00 UTC Jun 27 13:00–15:00 UTC Jun 28 18:00–19:00 UTC Jun 29 19:00–21:00, 23:00–24:00 UTC Jun 30 19:00–24:00 UTC
July
Jul 1 19:00–24:00 UTC Jul 2 19:00–23:00 UTC Jul 3 20:00–23:00 UTC Jul 4 14:00–17:00, 19:00–23:00 UTC Jul 5 18:00–21:00 UTC Jul 6 20:00–24:00 UTC Jul 7 00:00–01:00, 19:00–24:00 UTC Jul 8 00:00–01:00, 19:00–24:00 UTC Jul 9 02:00–03:00, 19:00–23:00 UTC Jul 10 00:00–01:00, 19:00–24:00 UTC Jul 11 00:00–01:00, 15:00–16:00, 19:00–22:00 UTC Jul 12 15:00–24:00 UTC Jul 13 00:00–01:00 UTC Jul 14 00:00–02:00, 19:00–22:00 UTC Jul 15 00:00–02:00, 20:00–24:00 UTC Jul 16 01:00–03:00, 20:00–24:00 UTC Jul 17 00:00–02:00, 21:00–24:00 UTC Jul 18 00:00–01:00, 14:00–16:00, 19:00–24:00 UTC Jul 19 00:00–02:00, 19:00–23:00 UTC Jul 20 22:00–24:00 UTC Jul 21 00:00–01:00, 21:00–24:00 UTC Jul 22 00:00–02:00, 21:00–24:00 UTC Jul 23 00:00–02:00 UTC Jul 24 00:00–02:00 UTC Jul 25 01:00–02:00, 17:00–23:00 UTC Jul 26 00:00–01:00, 17:00–24:00 UTC Jul 27 00:00–02:00 UTC