Most often in March, 24 days.
Highest follower count seen that year: 22,424.
January
Jan 1 19:00–22:00 UTC Jan 4 19:00–23:00 UTC Jan 5 20:00–21:00 UTC Jan 7 18:00–19:00, 21:00–22:00 UTC Jan 8 19:00–22:00 UTC Jan 9 18:00–19:00, 21:00–22:00 UTC Jan 11 19:00–22:00 UTC Jan 12 19:00–22:00 UTC Jan 13 21:00–24:00 UTC Jan 14 21:00–22:00 UTC Jan 16 18:00–21:00 UTC Jan 18 18:00–23:00 UTC Jan 19 18:00–23:00 UTC Jan 22 18:00–23:00 UTC Jan 24 20:00–22:00 UTC Jan 25 19:00–20:00 UTC Jan 27 19:00–21:00 UTC Jan 28 19:00–23:00 UTC Jan 29 20:00–21:00 UTC Jan 30 00:00–01:00 UTC Jan 31 00:00–01:00 UTC
February
Feb 1 19:00–20:00 UTC Feb 3 19:00–23:00 UTC Feb 4 19:00–22:00 UTC Feb 5 18:00–21:00 UTC Feb 6 01:00–02:00 UTC Feb 7 20:00–21:00 UTC Feb 8 18:00–21:00 UTC Feb 9 19:00–23:00 UTC Feb 10 21:00–22:00 UTC Feb 12 19:00–22:00 UTC Feb 13 00:00–01:00, 22:00–24:00 UTC Feb 14 19:00–21:00 UTC Feb 15 19:00–22:00 UTC Feb 18 19:00–21:00, 23:00–24:00 UTC Feb 20 20:00–24:00 UTC Feb 22 19:00–24:00 UTC Feb 23 18:00–23:00 UTC Feb 25 19:00–21:00 UTC Feb 26 18:00–20:00, 22:00–23:00 UTC Feb 28 17:00–18:00 UTC
March
Mar 1 19:00–21:00 UTC Mar 3 20:00–24:00 UTC Mar 4 00:00–01:00, 20:00–22:00 UTC Mar 6 20:00–24:00 UTC Mar 8 21:00–22:00 UTC Mar 9 18:00–24:00 UTC Mar 10 20:00–21:00 UTC Mar 11 19:00–20:00, 22:00–23:00 UTC Mar 12 20:00–24:00 UTC Mar 13 00:00–01:00, 18:00–23:00 UTC Mar 15 22:00–24:00 UTC Mar 16 19:00–21:00 UTC Mar 17 00:00–01:00 UTC Mar 18 18:00–23:00 UTC Mar 20 19:00–24:00 UTC Mar 22 19:00–23:00 UTC Mar 23 19:00–22:00 UTC Mar 24 18:00–20:00 UTC Mar 26 19:00–21:00 UTC Mar 27 21:00–23:00 UTC Mar 28 18:00–19:00 UTC Mar 29 20:00–21:00 UTC Mar 30 21:00–23:00 UTC Mar 31 21:00–22:00 UTC
April
Apr 1 21:00–24:00 UTC Apr 5 18:00–22:00 UTC Apr 6 19:00–22:00 UTC Apr 7 21:00–23:00 UTC Apr 9 00:00–01:00 UTC Apr 10 21:00–22:00 UTC Apr 12 18:00–19:00 UTC Apr 14 18:00–20:00 UTC Apr 19 19:00–22:00 UTC Apr 20 18:00–20:00, 22:00–23:00 UTC Apr 21 19:00–20:00 UTC Apr 23 11:00–12:00, 18:00–23:00 UTC Apr 24 20:00–21:00 UTC Apr 26 18:00–19:00, 21:00–22:00 UTC Apr 27 20:00–24:00 UTC Apr 28 23:00–24:00 UTC Apr 30 18:00–19:00 UTC
May
May 1 19:00–23:00 UTC May 3 19:00–21:00, 23:00–24:00 UTC May 4 19:00–24:00 UTC May 5 19:00–21:00 UTC May 6 19:00–20:00 UTC May 7 20:00–21:00 UTC May 8 19:00–22:00 UTC May 9 19:00–21:00 UTC May 11 19:00–20:00 UTC May 12 19:00–22:00 UTC May 13 19:00–23:00 UTC May 15 19:00–21:00 UTC May 16 21:00–22:00 UTC May 18 18:00–22:00 UTC May 19 19:00–23:00 UTC May 22 19:00–21:00 UTC May 24 17:00–23:00 UTC May 25 20:00–21:00 UTC May 26 20:00–23:00 UTC May 27 21:00–24:00 UTC May 28 18:00–20:00 UTC May 29 19:00–21:00 UTC May 30 20:00–22:00 UTC May 31 19:00–23:00 UTC
June
Jun 2 18:00–23:00 UTC Jun 4 18:00–22:00 UTC Jun 6 18:00–20:00 UTC Jun 7 19:00–23:00 UTC Jun 8 20:00–21:00 UTC Jun 9 19:00–24:00 UTC Jun 10 00:00–01:00, 20:00–24:00 UTC Jun 11 20:00–21:00 UTC Jun 12 19:00–20:00 UTC Jun 15 18:00–24:00 UTC Jun 16 19:00–21:00 UTC Jun 17 18:00–22:00 UTC Jun 18 20:00–21:00 UTC Jun 21 18:00–23:00 UTC Jun 22 19:00–21:00, 23:00–24:00 UTC Jun 23 20:00–23:00 UTC Jun 25 20:00–23:00 UTC Jun 26 19:00–21:00 UTC Jun 29 19:00–20:00, 22:00–23:00 UTC Jun 30 19:00–21:00, 23:00–24:00 UTC
July
Jul 2 20:00–21:00 UTC Jul 3 21:00–22:00 UTC Jul 5 19:00–23:00 UTC Jul 6 20:00–23:00 UTC Jul 7 19:00–20:00 UTC Jul 9 20:00–24:00 UTC Jul 10 00:00–01:00 UTC Jul 11 19:00–20:00 UTC Jul 12 18:00–23:00 UTC Jul 13 19:00–20:00, 22:00–23:00 UTC Jul 14 19:00–20:00 UTC Jul 15 20:00–22:00 UTC Jul 16 19:00–23:00 UTC Jul 18 19:00–20:00 UTC Jul 20 20:00–24:00 UTC Jul 21 20:00–21:00 UTC Jul 22 21:00–22:00 UTC Jul 23 21:00–22:00 UTC Jul 25 21:00–23:00 UTC Jul 26 19:00–20:00, 21:00–22:00 UTC Jul 27 20:00–21:00 UTC