Most often in April, 27 days.
Highest follower count seen that year: 16,798.
January
Jan 1 21:00–24:00 UTC Jan 2 00:00–01:00, 13:00–16:00, 19:00–24:00 UTC Jan 3 12:00–14:00, 19:00–23:00 UTC Jan 5 20:00–23:00 UTC Jan 6 19:00–24:00 UTC Jan 7 00:00–01:00, 20:00–23:00 UTC Jan 8 20:00–23:00 UTC Jan 9 00:00–01:00, 20:00–23:00 UTC Jan 10 12:00–15:00, 20:00–24:00 UTC Jan 11 00:00–02:00, 12:00–17:00 UTC Jan 12 12:00–15:00, 20:00–23:00 UTC Jan 13 20:00–23:00 UTC Jan 14 12:00–16:00, 20:00–24:00 UTC Jan 16 12:00–15:00, 20:00–24:00 UTC Jan 17 00:00–01:00, 20:00–23:00 UTC Jan 19 20:00–24:00 UTC Jan 20 00:00–01:00, 20:00–24:00 UTC Jan 21 20:00–23:00 UTC Jan 23 14:00–16:00, 21:00–23:00 UTC Jan 24 19:00–23:00 UTC Jan 25 19:00–20:00, 22:00–23:00 UTC Jan 26 19:00–24:00 UTC Jan 28 20:00–24:00 UTC Jan 29 00:00–01:00, 20:00–22:00 UTC Jan 30 13:00–15:00, 20:00–24:00 UTC Jan 31 20:00–24:00 UTC
February
Feb 1 00:00–01:00, 19:00–24:00 UTC Feb 2 12:00–14:00, 22:00–23:00 UTC Feb 3 20:00–24:00 UTC Feb 4 20:00–23:00 UTC Feb 11 20:00–24:00 UTC Feb 12 20:00–24:00 UTC Feb 13 19:00–23:00 UTC Feb 14 19:00–24:00 UTC Feb 15 00:00–01:00, 20:00–23:00 UTC Feb 16 19:00–24:00 UTC Feb 17 00:00–01:00, 20:00–24:00 UTC Feb 18 19:00–22:00 UTC Feb 20 20:00–24:00 UTC Feb 21 00:00–01:00, 19:00–21:00 UTC Feb 23 19:00–24:00 UTC Feb 24 00:00–01:00, 20:00–24:00 UTC Feb 25 19:00–24:00 UTC Feb 27 20:00–24:00 UTC Feb 28 00:00–01:00, 20:00–24:00 UTC
March
Mar 1 00:00–01:00, 19:00–24:00 UTC Mar 2 20:00–24:00 UTC Mar 3 20:00–24:00 UTC Mar 4 00:00–01:00, 20:00–22:00 UTC Mar 7 00:00–01:00 UTC Mar 9 22:00–24:00 UTC Mar 10 00:00–01:00, 12:00–15:00, 20:00–24:00 UTC Mar 11 00:00–01:00, 13:00–15:00, 20:00–23:00 UTC Mar 12 13:00–15:00, 21:00–24:00 UTC Mar 13 12:00–16:00 UTC Mar 16 12:00–15:00, 19:00–22:00 UTC Mar 17 12:00–14:00, 21:00–22:00 UTC Mar 18 13:00–15:00, 20:00–24:00 UTC Mar 19 21:00–24:00 UTC Mar 20 20:00–24:00 UTC Mar 21 12:00–15:00, 20:00–24:00 UTC Mar 22 12:00–17:00 UTC Mar 23 19:00–24:00 UTC Mar 24 12:00–15:00, 19:00–24:00 UTC Mar 25 20:00–24:00 UTC Mar 27 13:00–15:00, 20:00–23:00 UTC Mar 28 12:00–16:00, 20:00–24:00 UTC Mar 30 20:00–24:00 UTC Mar 31 21:00–23:00 UTC
April
Apr 1 21:00–24:00 UTC Apr 2 00:00–02:00, 19:00–24:00 UTC Apr 3 00:00–01:00 UTC Apr 4 20:00–24:00 UTC Apr 5 00:00–01:00, 12:00–16:00 UTC Apr 6 11:00–14:00, 20:00–21:00 UTC Apr 8 21:00–22:00 UTC Apr 9 00:00–01:00, 23:00–24:00 UTC Apr 10 00:00–01:00, 20:00–22:00 UTC Apr 11 19:00–23:00 UTC Apr 12 20:00–22:00 UTC Apr 13 20:00–24:00 UTC Apr 14 00:00–01:00, 19:00–23:00 UTC Apr 15 19:00–23:00 UTC Apr 16 00:00–01:00 UTC Apr 17 20:00–24:00 UTC Apr 18 12:00–16:00, 19:00–24:00 UTC Apr 19 12:00–17:00 UTC Apr 20 12:00–15:00, 23:00–24:00 UTC Apr 21 00:00–02:00, 19:00–23:00 UTC Apr 22 19:00–24:00 UTC Apr 23 19:00–24:00 UTC Apr 24 20:00–24:00 UTC Apr 25 20:00–24:00 UTC Apr 26 00:00–01:00, 12:00–16:00 UTC Apr 27 12:00–15:00 UTC Apr 30 20:00–24:00 UTC
May
May 1 13:00–15:00, 20:00–23:00 UTC May 2 15:00–16:00, 19:00–20:00 UTC May 3 13:00–20:00 UTC May 4 12:00–15:00, 19:00–23:00 UTC May 6 19:00–24:00 UTC May 7 00:00–01:00, 20:00–21:00 UTC May 8 13:00–15:00, 20:00–24:00 UTC May 9 20:00–24:00 UTC May 11 12:00–15:00, 19:00–24:00 UTC May 12 20:00–24:00 UTC May 13 19:00–24:00 UTC May 15 19:00–23:00 UTC May 16 19:00–22:00 UTC May 17 12:00–16:00 UTC May 23 21:00–24:00 UTC May 24 19:00–24:00 UTC May 25 12:00–16:00 UTC May 26 13:00–17:00 UTC May 27 19:00–23:00 UTC May 28 12:00–17:00 UTC May 29 12:00–17:00 UTC May 30 12:00–18:00 UTC May 31 12:00–16:00 UTC
June
Jun 1 19:00–24:00 UTC Jun 2 12:00–16:00 UTC Jun 3 13:00–16:00 UTC Jun 4 12:00–16:00 UTC Jun 5 12:00–18:00 UTC Jun 9 20:00–24:00 UTC Jun 10 00:00–02:00, 12:00–18:00 UTC Jun 11 20:00–24:00 UTC Jun 12 00:00–01:00, 12:00–16:00, 20:00–24:00 UTC Jun 13 00:00–01:00, 12:00–15:00 UTC Jun 15 11:00–19:00 UTC Jun 16 12:00–16:00 UTC Jun 18 12:00–15:00, 19:00–24:00 UTC Jun 19 00:00–01:00, 12:00–16:00, 19:00–23:00 UTC Jun 20 12:00–16:00, 19:00–24:00 UTC Jun 21 12:00–14:00, 19:00–22:00 UTC Jun 22 19:00–24:00 UTC Jun 23 00:00–01:00, 19:00–21:00, 23:00–24:00 UTC Jun 24 19:00–22:00 UTC Jun 26 19:00–24:00 UTC Jun 27 00:00–01:00, 22:00–24:00 UTC Jun 29 19:00–23:00 UTC Jun 30 20:00–24:00 UTC
July
Jul 1 12:00–17:00 UTC Jul 2 19:00–24:00 UTC Jul 3 12:00–16:00, 20:00–23:00 UTC Jul 4 12:00–14:00, 19:00–22:00 UTC Jul 6 19:00–24:00 UTC Jul 7 00:00–01:00, 19:00–23:00 UTC Jul 8 19:00–24:00 UTC Jul 23 13:00–19:00 UTC Jul 24 12:00–17:00, 20:00–23:00 UTC Jul 25 02:00–03:00, 12:00–15:00, 20:00–24:00 UTC Jul 26 16:00–21:00 UTC Jul 27 20:00–21:00 UTC