Most often in February, 28 days.
Highest follower count seen that year: 2,531.
January
Jan 27 21:00–24:00 UTC Jan 28 00:00–03:00, 21:00–24:00 UTC Jan 29 00:00–03:00, 21:00–24:00 UTC Jan 30 00:00–02:00, 20:00–24:00 UTC Jan 31 00:00–03:00 UTC
February
Feb 1 20:00–24:00 UTC Feb 2 00:00–02:00, 21:00–23:00 UTC Feb 3 00:00–02:00, 20:00–24:00 UTC Feb 4 00:00–02:00 UTC Feb 5 21:00–24:00 UTC Feb 6 00:00–03:00, 22:00–23:00 UTC Feb 7 00:00–03:00, 21:00–24:00 UTC Feb 8 00:00–03:00 UTC Feb 9 21:00–24:00 UTC Feb 10 01:00–02:00, 20:00–24:00 UTC Feb 11 00:00–02:00, 20:00–24:00 UTC Feb 12 00:00–03:00 UTC Feb 13 20:00–24:00 UTC Feb 14 00:00–02:00, 21:00–24:00 UTC Feb 15 00:00–02:00, 21:00–24:00 UTC Feb 16 00:00–02:00, 21:00–24:00 UTC Feb 17 00:00–02:00, 21:00–24:00 UTC Feb 18 00:00–02:00, 20:00–24:00 UTC Feb 19 00:00–03:00, 21:00–24:00 UTC Feb 20 00:00–03:00, 21:00–24:00 UTC Feb 21 00:00–04:00, 22:00–24:00 UTC Feb 22 00:00–01:00 UTC Feb 23 20:00–24:00 UTC Feb 24 00:00–03:00, 21:00–24:00 UTC Feb 25 00:00–02:00 UTC Feb 26 21:00–24:00 UTC Feb 27 00:00–02:00, 20:00–24:00 UTC Feb 28 00:00–02:00, 20:00–24:00 UTC
March
Mar 1 00:00–03:00, 19:00–21:00 UTC Mar 2 21:00–24:00 UTC Mar 3 00:00–02:00, 21:00–24:00 UTC Mar 4 00:00–03:00, 20:00–22:00 UTC Mar 5 00:00–02:00 UTC Mar 9 22:00–24:00 UTC Mar 10 00:00–02:00, 21:00–24:00 UTC Mar 11 00:00–03:00, 21:00–24:00 UTC Mar 12 00:00–03:00, 21:00–24:00 UTC Mar 13 00:00–03:00 UTC Mar 14 15:00–23:00 UTC Mar 15 17:00–20:00 UTC Mar 16 22:00–24:00 UTC Mar 17 00:00–02:00, 21:00–24:00 UTC Mar 18 01:00–02:00, 23:00–24:00 UTC Mar 19 00:00–03:00, 21:00–24:00 UTC Mar 20 00:00–01:00, 21:00–24:00 UTC Mar 21 00:00–01:00, 22:00–24:00 UTC Mar 22 00:00–02:00 UTC Mar 25 21:00–24:00 UTC Mar 26 00:00–02:00, 21:00–24:00 UTC Mar 27 21:00–24:00 UTC Mar 28 00:00–02:00, 17:00–21:00 UTC Mar 29 13:00–19:00 UTC Mar 30 21:00–24:00 UTC Mar 31 00:00–03:00, 21:00–24:00 UTC
April
Apr 1 00:00–02:00, 21:00–24:00 UTC Apr 2 00:00–02:00, 20:00–24:00 UTC Apr 3 00:00–01:00 UTC Apr 4 21:00–24:00 UTC Apr 5 00:00–02:00, 14:00–21:00 UTC Apr 7 22:00–24:00 UTC Apr 8 00:00–02:00, 21:00–24:00 UTC Apr 9 00:00–02:00 UTC Apr 12 15:00–21:00 UTC Apr 14 20:00–24:00 UTC Apr 15 00:00–03:00, 21:00–24:00 UTC Apr 16 00:00–03:00, 20:00–24:00 UTC Apr 17 00:00–02:00, 21:00–24:00 UTC Apr 18 00:00–03:00 UTC Apr 19 14:00–19:00 UTC Apr 20 21:00–24:00 UTC Apr 21 00:00–02:00, 21:00–24:00 UTC Apr 22 00:00–02:00 UTC Apr 23 21:00–24:00 UTC Apr 24 00:00–02:00 UTC Apr 26 13:00–18:00 UTC Apr 27 22:00–24:00 UTC Apr 28 00:00–02:00, 21:00–24:00 UTC Apr 29 00:00–03:00, 22:00–24:00 UTC Apr 30 00:00–02:00, 22:00–24:00 UTC
May
May 1 00:00–03:00 UTC May 2 22:00–24:00 UTC May 3 01:00–02:00, 16:00–18:00 UTC May 4 21:00–24:00 UTC May 5 00:00–02:00, 22:00–24:00 UTC May 6 00:00–02:00 UTC May 8 22:00–24:00 UTC May 9 21:00–24:00 UTC May 10 00:00–01:00 UTC May 11 22:00–24:00 UTC May 12 00:00–02:00, 22:00–24:00 UTC May 13 01:00–02:00, 22:00–24:00 UTC May 14 00:00–01:00 UTC May 15 22:00–23:00 UTC May 16 00:00–02:00 UTC May 20 21:00–22:00 UTC May 21 21:00–24:00 UTC May 22 00:00–03:00, 22:00–24:00 UTC May 23 00:00–02:00, 18:00–19:00, 21:00–22:00 UTC May 25 21:00–24:00 UTC May 26 00:00–02:00, 21:00–24:00 UTC May 27 00:00–02:00, 23:00–24:00 UTC May 28 00:00–02:00, 22:00–24:00 UTC May 29 00:00–02:00 UTC May 30 21:00–24:00 UTC May 31 00:00–02:00 UTC
June
Jun 1 22:00–24:00 UTC Jun 2 00:00–03:00, 22:00–24:00 UTC Jun 3 00:00–02:00 UTC Jun 4 23:00–24:00 UTC Jun 5 00:00–04:00, 22:00–24:00 UTC Jun 6 00:00–03:00 UTC Jun 7 19:00–22:00 UTC Jun 8 22:00–24:00 UTC Jun 9 00:00–03:00, 22:00–24:00 UTC Jun 10 00:00–03:00, 22:00–24:00 UTC Jun 11 00:00–02:00, 23:00–24:00 UTC Jun 12 00:00–03:00 UTC Jun 16 21:00–24:00 UTC Jun 17 00:00–02:00 UTC Jun 18 21:00–24:00 UTC Jun 19 00:00–02:00, 22:00–23:00 UTC Jun 20 00:00–02:00, 22:00–24:00 UTC Jun 21 00:00–02:00 UTC Jun 25 00:00–02:00, 23:00–24:00 UTC Jun 26 00:00–02:00, 22:00–24:00 UTC Jun 27 00:00–02:00 UTC
July
Jul 1 21:00–24:00 UTC Jul 2 00:00–01:00, 21:00–24:00 UTC Jul 3 00:00–02:00, 22:00–24:00 UTC Jul 4 00:00–01:00 UTC Jul 6 22:00–24:00 UTC Jul 7 00:00–02:00 UTC Jul 8 23:00–24:00 UTC Jul 9 00:00–03:00, 22:00–24:00 UTC Jul 10 00:00–03:00, 22:00–24:00 UTC Jul 11 00:00–03:00 UTC Jul 15 23:00–24:00 UTC Jul 16 00:00–03:00 UTC Jul 18 00:00–05:00 UTC