Most often in March, 28 days.
Highest follower count seen that year: 6,150.
January
Jan 2 12:00–19:00 UTC Jan 3 12:00–19:00 UTC Jan 4 18:00–23:00 UTC Jan 5 12:00–19:00 UTC Jan 7 11:00–19:00 UTC Jan 8 12:00–19:00 UTC Jan 9 14:00–19:00 UTC Jan 10 12:00–20:00 UTC Jan 11 17:00–22:00 UTC Jan 12 12:00–19:00 UTC Jan 14 12:00–20:00 UTC Jan 15 20:00–24:00 UTC Jan 16 00:00–03:00 UTC Jan 17 12:00–19:00 UTC Jan 18 18:00–23:00 UTC Jan 19 11:00–18:00 UTC Jan 21 12:00–19:00 UTC Jan 22 11:00–18:00 UTC Jan 23 12:00–18:00 UTC Jan 24 12:00–19:00 UTC Jan 25 12:00–17:00 UTC Jan 26 12:00–19:00 UTC Jan 27 11:00–18:00 UTC Jan 28 11:00–19:00 UTC Jan 29 12:00–19:00 UTC Jan 30 18:00–24:00 UTC Jan 31 00:00–02:00, 12:00–20:00 UTC
February
Feb 1 18:00–23:00 UTC Feb 2 12:00–19:00 UTC Feb 4 12:00–20:00 UTC Feb 5 12:00–20:00 UTC Feb 6 12:00–20:00 UTC Feb 7 12:00–19:00 UTC Feb 8 18:00–23:00 UTC Feb 9 12:00–20:00 UTC Feb 11 14:00–21:00 UTC Feb 12 12:00–19:00 UTC Feb 13 12:00–19:00 UTC Feb 14 13:00–20:00 UTC Feb 15 12:00–17:00 UTC Feb 16 12:00–19:00 UTC Feb 18 12:00–20:00 UTC Feb 19 12:00–20:00 UTC Feb 21 12:00–20:00 UTC Feb 22 12:00–17:00 UTC Feb 23 13:00–20:00 UTC Feb 24 12:00–20:00 UTC Feb 25 19:00–24:00 UTC Feb 26 00:00–03:00, 13:00–20:00 UTC Feb 27 12:00–19:00 UTC
March
Mar 1 17:00–21:00 UTC Mar 2 12:00–18:00 UTC Mar 4 12:00–19:00 UTC Mar 5 12:00–20:00 UTC Mar 6 11:00–19:00 UTC Mar 7 19:00–24:00 UTC Mar 8 00:00–02:00, 12:00–17:00 UTC Mar 9 12:00–20:00 UTC Mar 11 12:00–20:00 UTC Mar 12 12:00–20:00 UTC Mar 13 11:00–19:00 UTC Mar 14 12:00–20:00 UTC Mar 15 18:00–24:00 UTC Mar 16 18:00–24:00 UTC Mar 18 20:00–24:00 UTC Mar 19 00:00–02:00, 12:00–19:00 UTC Mar 20 12:00–18:00 UTC Mar 21 12:00–19:00 UTC Mar 22 12:00–17:00 UTC Mar 23 20:00–24:00 UTC Mar 24 00:00–02:00 UTC Mar 25 20:00–24:00 UTC Mar 26 00:00–02:00, 13:00–20:00 UTC Mar 27 12:00–20:00 UTC Mar 28 19:00–24:00 UTC Mar 29 00:00–03:00, 17:00–23:00 UTC Mar 30 11:00–19:00 UTC Mar 31 12:00–20:00 UTC
April
Apr 1 18:00–24:00 UTC Apr 2 00:00–01:00, 18:00–24:00 UTC Apr 4 18:00–24:00 UTC Apr 5 00:00–01:00, 16:00–22:00 UTC Apr 7 18:00–24:00 UTC Apr 8 00:00–01:00, 21:00–23:00 UTC Apr 9 00:00–03:00, 23:00–24:00 UTC Apr 10 00:00–01:00, 19:00–22:00 UTC Apr 11 18:00–24:00 UTC Apr 12 00:00–02:00, 16:00–21:00 UTC Apr 14 19:00–24:00 UTC Apr 15 00:00–01:00, 19:00–24:00 UTC Apr 16 00:00–04:00, 19:00–24:00 UTC Apr 17 00:00–02:00, 20:00–24:00 UTC Apr 18 00:00–03:00, 13:00–20:00 UTC Apr 19 18:00–23:00 UTC Apr 21 18:00–24:00 UTC Apr 22 00:00–01:00, 18:00–24:00 UTC Apr 23 00:00–01:00, 19:00–24:00 UTC Apr 24 00:00–01:00, 19:00–24:00 UTC Apr 25 00:00–01:00, 20:00–24:00 UTC Apr 26 00:00–03:00, 17:00–22:00 UTC Apr 28 19:00–24:00 UTC Apr 29 00:00–02:00, 19:00–24:00 UTC Apr 30 00:00–03:00, 19:00–24:00 UTC
May
May 1 00:00–02:00, 18:00–24:00 UTC May 2 00:00–01:00, 19:00–20:00, 23:00–24:00 UTC May 3 00:00–03:00, 18:00–22:00 UTC May 5 19:00–24:00 UTC May 6 00:00–02:00, 20:00–24:00 UTC May 7 00:00–03:00, 20:00–21:00 UTC May 8 19:00–24:00 UTC May 9 12:00–19:00 UTC May 10 17:00–23:00 UTC May 12 20:00–24:00 UTC May 13 00:00–02:00, 19:00–24:00 UTC May 14 00:00–01:00, 19:00–24:00 UTC May 15 19:00–23:00 UTC May 16 01:00–02:00, 18:00–24:00 UTC May 17 00:00–01:00, 18:00–22:00 UTC May 19 12:00–19:00 UTC May 20 18:00–22:00 UTC May 21 19:00–24:00 UTC May 22 00:00–01:00, 18:00–24:00 UTC May 23 21:00–24:00 UTC May 24 00:00–02:00, 17:00–22:00 UTC May 26 19:00–24:00 UTC May 27 00:00–03:00, 19:00–24:00 UTC May 28 00:00–02:00, 19:00–24:00 UTC May 29 00:00–03:00, 17:00–22:00 UTC May 30 20:00–24:00 UTC May 31 00:00–02:00, 15:00–21:00 UTC
June
Jun 2 12:00–19:00 UTC Jun 3 19:00–24:00 UTC Jun 4 00:00–03:00, 19:00–24:00 UTC Jun 5 00:00–02:00, 19:00–24:00 UTC Jun 6 00:00–02:00, 19:00–24:00 UTC Jun 7 00:00–04:00, 17:00–22:00 UTC Jun 8 12:00–18:00 UTC Jun 10 20:00–24:00 UTC Jun 11 00:00–03:00, 18:00–24:00 UTC Jun 12 00:00–02:00, 19:00–24:00 UTC Jun 13 00:00–02:00, 13:00–19:00 UTC Jun 15 19:00–24:00 UTC Jun 16 00:00–01:00 UTC