Most often in January, 24 days.
Highest follower count seen that year: 18,848.
January
Jan 3 22:00–24:00 UTC Jan 5 15:00–17:00, 20:00–24:00 UTC Jan 6 00:00–01:00, 15:00–16:00, 18:00–23:00 UTC Jan 7 15:00–17:00, 20:00–24:00 UTC Jan 8 00:00–01:00, 14:00–23:00 UTC Jan 9 20:00–24:00 UTC Jan 10 00:00–04:00, 16:00–24:00 UTC Jan 11 00:00–02:00, 15:00–22:00 UTC Jan 12 16:00–20:00, 23:00–24:00 UTC Jan 13 00:00–03:00, 21:00–24:00 UTC Jan 14 15:00–18:00 UTC Jan 15 01:00–04:00, 12:00–14:00 UTC Jan 16 13:00–18:00 UTC Jan 17 09:00–11:00 UTC Jan 21 00:00–03:00, 21:00–24:00 UTC Jan 22 00:00–04:00, 16:00–24:00 UTC Jan 23 00:00–02:00, 15:00–24:00 UTC Jan 24 00:00–02:00, 10:00–12:00 UTC Jan 25 16:00–20:00, 22:00–24:00 UTC Jan 26 17:00–24:00 UTC Jan 27 00:00–02:00, 16:00–21:00 UTC Jan 28 04:00–05:00, 15:00–24:00 UTC Jan 29 00:00–01:00, 03:00–04:00, 15:00–24:00 UTC Jan 30 00:00–04:00, 09:00–16:00 UTC
February
Feb 2 16:00–17:00 UTC Feb 3 16:00–24:00 UTC Feb 4 00:00–02:00, 23:00–24:00 UTC Feb 5 00:00–01:00, 19:00–24:00 UTC Feb 6 17:00–18:00, 20:00–21:00, 23:00–24:00 UTC Feb 7 00:00–03:00, 16:00–20:00, 22:00–24:00 UTC Feb 8 00:00–02:00 UTC Feb 9 16:00–17:00 UTC Feb 11 15:00–18:00, 20:00–24:00 UTC Feb 12 00:00–02:00, 10:00–16:00 UTC Feb 13 12:00–13:00, 17:00–23:00 UTC Feb 14 10:00–12:00 UTC Feb 16 13:00–22:00 UTC Feb 17 12:00–19:00 UTC Feb 18 12:00–17:00, 19:00–24:00 UTC Feb 19 00:00–01:00, 16:00–17:00, 20:00–23:00 UTC Feb 20 20:00–23:00 UTC Feb 21 00:00–02:00, 09:00–13:00 UTC Feb 24 13:00–16:00 UTC Feb 25 15:00–21:00 UTC Feb 26 15:00–17:00, 21:00–24:00 UTC Feb 27 00:00–03:00, 10:00–18:00, 21:00–24:00 UTC Feb 28 00:00–04:00, 10:00–17:00 UTC
March
Mar 1 01:00–04:00 UTC Mar 2 17:00–23:00 UTC Mar 4 16:00–18:00, 20:00–22:00 UTC Mar 6 17:00–20:00 UTC Mar 7 00:00–02:00, 10:00–14:00, 16:00–17:00 UTC Mar 9 00:00–04:00, 16:00–19:00 UTC Mar 10 10:00–17:00 UTC Mar 12 13:00–15:00, 21:00–24:00 UTC Mar 13 00:00–02:00, 15:00–24:00 UTC Mar 16 16:00–22:00 UTC Mar 18 22:00–24:00 UTC Mar 19 00:00–02:00, 15:00–19:00 UTC Mar 20 23:00–24:00 UTC Mar 21 01:00–03:00, 09:00–12:00 UTC Mar 23 14:00–23:00 UTC Mar 24 16:00–23:00 UTC Mar 25 15:00–24:00 UTC Mar 26 16:00–17:00, 21:00–24:00 UTC Mar 27 00:00–02:00, 14:00–24:00 UTC Mar 28 00:00–02:00 UTC Mar 30 17:00–24:00 UTC Mar 31 16:00–18:00 UTC
April
Apr 1 00:00–02:00 UTC Apr 4 15:00–18:00 UTC Apr 5 18:00–24:00 UTC Apr 6 00:00–01:00, 12:00–16:00 UTC Apr 8 16:00–17:00 UTC Apr 9 16:00–17:00 UTC Apr 10 14:00–21:00 UTC Apr 11 10:00–13:00 UTC Apr 14 14:00–16:00, 22:00–24:00 UTC Apr 15 00:00–02:00, 15:00–17:00, 19:00–22:00 UTC Apr 16 23:00–24:00 UTC Apr 17 00:00–02:00, 15:00–24:00 UTC Apr 18 00:00–04:00, 10:00–13:00 UTC Apr 19 14:00–17:00 UTC Apr 20 12:00–21:00 UTC Apr 21 14:00–15:00, 17:00–20:00 UTC Apr 23 11:00–14:00 UTC Apr 27 18:00–24:00 UTC Apr 28 11:00–20:00 UTC Apr 29 13:00–16:00 UTC Apr 30 13:00–18:00 UTC
May
May 1 14:00–16:00, 18:00–22:00 UTC May 4 19:00–20:00 UTC May 5 18:00–21:00 UTC May 6 11:00–17:00, 20:00–24:00 UTC May 7 13:00–16:00 UTC May 9 11:00–19:00 UTC May 11 21:00–22:00 UTC May 12 12:00–17:00 UTC May 13 21:00–23:00 UTC May 14 13:00–17:00, 19:00–20:00 UTC May 16 16:00–17:00 UTC May 19 18:00–23:00 UTC May 20 11:00–20:00 UTC May 21 11:00–13:00 UTC May 23 09:00–12:00 UTC May 27 13:00–17:00, 21:00–24:00 UTC May 28 00:00–01:00, 12:00–17:00, 19:00–21:00 UTC May 29 11:00–17:00, 19:00–22:00 UTC May 30 09:00–13:00 UTC
June
Jun 4 08:00–14:00 UTC Jun 5 11:00–13:00, 15:00–19:00 UTC Jun 6 10:00–12:00 UTC Jun 8 14:00–15:00, 19:00–24:00 UTC Jun 9 00:00–01:00, 23:00–24:00 UTC Jun 10 00:00–01:00, 14:00–19:00 UTC Jun 11 13:00–17:00, 19:00–24:00 UTC Jun 12 00:00–03:00, 10:00–18:00, 21:00–24:00 UTC Jun 13 11:00–14:00 UTC Jun 15 12:00–16:00, 20:00–22:00 UTC Jun 16 12:00–16:00, 19:00–21:00 UTC Jun 17 00:00–01:00 UTC Jun 18 12:00–22:00 UTC Jun 20 09:00–13:00 UTC Jun 22 18:00–23:00 UTC Jun 23 13:00–18:00, 22:00–24:00 UTC Jun 24 00:00–01:00, 21:00–24:00 UTC Jun 25 14:00–17:00, 19:00–22:00 UTC Jun 27 10:00–12:00 UTC Jun 30 13:00–16:00 UTC
July
Jul 4 17:00–18:00 UTC Jul 8 15:00–17:00, 19:00–24:00 UTC Jul 10 13:00–20:00 UTC Jul 22 20:00–24:00 UTC Jul 23 19:00–21:00, 23:00–24:00 UTC Jul 24 00:00–03:00, 19:00–21:00 UTC Jul 25 01:00–02:00, 14:00–17:00, 21:00–22:00 UTC