Most often in March, 31 days.
Highest follower count seen that year: 39,661.
January
Jan 2 11:00–16:00 UTC Jan 3 10:00–16:00 UTC Jan 4 11:00–17:00 UTC Jan 5 11:00–17:00 UTC Jan 6 11:00–17:00 UTC Jan 7 11:00–17:00 UTC Jan 8 10:00–17:00, 20:00–23:00 UTC Jan 9 12:00–18:00, 20:00–23:00 UTC Jan 10 10:00–17:00 UTC Jan 11 19:00–23:00 UTC Jan 12 11:00–16:00 UTC Jan 13 11:00–17:00 UTC Jan 14 11:00–16:00 UTC Jan 15 10:00–15:00 UTC Jan 16 11:00–17:00, 20:00–24:00 UTC Jan 17 00:00–01:00, 19:00–24:00 UTC Jan 18 13:00–17:00 UTC Jan 19 12:00–17:00 UTC Jan 20 09:00–14:00 UTC Jan 21 12:00–17:00 UTC Jan 22 19:00–24:00 UTC Jan 23 00:00–01:00, 17:00–23:00 UTC Jan 24 17:00–23:00 UTC Jan 25 18:00–20:00, 22:00–24:00 UTC Jan 26 15:00–23:00 UTC Jan 27 12:00–17:00, 19:00–22:00 UTC Jan 28 18:00–22:00 UTC Jan 29 11:00–16:00 UTC Jan 30 17:00–23:00 UTC Jan 31 16:00–22:00 UTC
February
Feb 2 15:00–20:00 UTC Feb 3 16:00–21:00 UTC Feb 4 15:00–19:00 UTC Feb 5 13:00–19:00 UTC Feb 6 13:00–14:00, 16:00–20:00 UTC Feb 7 16:00–21:00 UTC Feb 8 11:00–18:00 UTC Feb 9 13:00–17:00, 19:00–23:00 UTC Feb 10 13:00–17:00 UTC Feb 11 13:00–18:00 UTC Feb 12 13:00–16:00, 19:00–23:00 UTC Feb 13 16:00–21:00 UTC Feb 14 18:00–21:00 UTC Feb 15 17:00–22:00 UTC Feb 17 18:00–19:00 UTC Feb 18 17:00–23:00 UTC Feb 19 15:00–20:00 UTC Feb 20 20:00–23:00 UTC Feb 21 15:00–20:00 UTC Feb 22 15:00–20:00 UTC Feb 23 18:00–23:00 UTC Feb 24 19:00–23:00 UTC Feb 25 11:00–15:00 UTC Feb 26 15:00–17:00 UTC Feb 27 22:00–23:00 UTC Feb 28 14:00–20:00 UTC
March
Mar 1 17:00–20:00 UTC Mar 2 14:00–21:00 UTC Mar 3 15:00–21:00 UTC Mar 4 18:00–22:00 UTC Mar 5 10:00–12:00, 23:00–24:00 UTC Mar 6 00:00–01:00, 21:00–24:00 UTC Mar 7 00:00–01:00, 20:00–23:00 UTC Mar 8 21:00–24:00 UTC Mar 9 00:00–02:00, 22:00–24:00 UTC Mar 10 00:00–02:00, 22:00–24:00 UTC Mar 11 00:00–02:00 UTC Mar 12 00:00–03:00, 17:00–21:00 UTC Mar 13 17:00–21:00 UTC Mar 14 16:00–22:00 UTC Mar 15 17:00–21:00 UTC Mar 16 18:00–23:00 UTC Mar 17 17:00–22:00 UTC Mar 18 17:00–22:00 UTC Mar 19 17:00–22:00 UTC Mar 20 17:00–22:00 UTC Mar 21 17:00–22:00 UTC Mar 22 17:00–22:00 UTC Mar 23 18:00–22:00 UTC Mar 24 18:00–22:00 UTC Mar 25 17:00–22:00 UTC Mar 26 18:00–23:00 UTC Mar 27 22:00–24:00 UTC Mar 28 00:00–03:00, 19:00–23:00 UTC Mar 29 18:00–23:00 UTC Mar 30 16:00–21:00 UTC Mar 31 16:00–22:00 UTC
April
Apr 1 16:00–20:00 UTC Apr 2 16:00–21:00 UTC Apr 3 16:00–17:00 UTC Apr 4 16:00–21:00 UTC Apr 5 18:00–23:00 UTC Apr 6 20:00–24:00 UTC Apr 7 17:00–21:00 UTC Apr 8 16:00–21:00 UTC Apr 9 16:00–20:00 UTC Apr 10 16:00–21:00 UTC Apr 11 16:00–21:00 UTC Apr 12 21:00–23:00 UTC Apr 13 16:00–21:00 UTC Apr 14 16:00–21:00 UTC Apr 15 18:00–23:00 UTC Apr 16 17:00–22:00 UTC Apr 17 19:00–22:00 UTC Apr 18 19:00–23:00 UTC Apr 19 17:00–21:00 UTC Apr 20 16:00–22:00 UTC Apr 21 17:00–21:00 UTC Apr 22 18:00–22:00 UTC Apr 23 17:00–21:00 UTC Apr 24 17:00–23:00 UTC Apr 25 20:00–24:00 UTC Apr 26 16:00–21:00 UTC Apr 27 17:00–23:00 UTC Apr 28 17:00–22:00 UTC Apr 29 18:00–21:00 UTC Apr 30 17:00–19:00 UTC
May
May 1 17:00–20:00 UTC May 2 19:00–20:00 UTC May 3 17:00–21:00 UTC May 4 15:00–20:00 UTC May 5 17:00–21:00 UTC May 6 17:00–20:00 UTC May 7 06:00–08:00, 16:00–20:00 UTC May 8 17:00–21:00 UTC May 9 15:00–18:00 UTC May 10 16:00–21:00 UTC May 11 17:00–20:00 UTC May 12 16:00–20:00 UTC May 13 15:00–19:00 UTC May 14 19:00–21:00 UTC May 15 19:00–21:00 UTC May 16 19:00–23:00 UTC May 17 19:00–23:00 UTC May 18 18:00–21:00 UTC May 19 18:00–21:00 UTC May 21 18:00–22:00 UTC May 22 18:00–21:00 UTC May 24 19:00–23:00 UTC May 25 16:00–20:00 UTC May 26 16:00–21:00 UTC May 27 16:00–21:00 UTC May 28 18:00–21:00 UTC May 29 19:00–23:00 UTC May 30 22:00–24:00 UTC May 31 00:00–01:00, 22:00–24:00 UTC
June
Jun 1 00:00–01:00, 21:00–24:00 UTC Jun 2 21:00–24:00 UTC Jun 3 21:00–23:00 UTC Jun 4 20:00–24:00 UTC Jun 5 21:00–24:00 UTC Jun 7 01:00–03:00, 18:00–23:00 UTC Jun 8 21:00–24:00 UTC Jun 9 20:00–23:00 UTC Jun 10 21:00–24:00 UTC Jun 11 21:00–23:00 UTC Jun 12 13:00–17:00 UTC Jun 13 15:00–17:00 UTC Jun 15 21:00–24:00 UTC Jun 16 19:00–22:00 UTC Jun 18 19:00–22:00 UTC Jun 19 17:00–21:00 UTC Jun 20 22:00–24:00 UTC Jun 21 00:00–01:00, 16:00–18:00, 20:00–21:00 UTC Jun 22 10:00–16:00 UTC Jun 23 11:00–15:00, 21:00–22:00 UTC Jun 24 21:00–23:00 UTC Jun 25 10:00–14:00, 19:00–21:00 UTC Jun 26 12:00–14:00, 21:00–23:00 UTC Jun 27 11:00–14:00, 20:00–23:00 UTC Jun 28 11:00–14:00, 18:00–21:00 UTC Jun 29 12:00–15:00, 20:00–23:00 UTC Jun 30 20:00–22:00 UTC
July
Jul 6 14:00–19:00 UTC Jul 7 15:00–19:00 UTC Jul 8 13:00–16:00 UTC Jul 9 14:00–16:00, 21:00–22:00 UTC Jul 10 14:00–16:00, 20:00–22:00 UTC Jul 11 13:00–15:00, 21:00–23:00 UTC Jul 12 19:00–21:00 UTC Jul 13 18:00–21:00 UTC Jul 14 12:00–14:00, 19:00–21:00 UTC Jul 15 14:00–16:00 UTC Jul 16 16:00–19:00 UTC Jul 17 13:00–17:00, 21:00–22:00 UTC Jul 18 12:00–15:00, 21:00–23:00 UTC Jul 19 12:00–15:00, 20:00–21:00 UTC Jul 20 16:00–20:00, 22:00–23:00 UTC Jul 21 00:00–01:00, 16:00–17:00 UTC