Most often in March, 27 days.
Highest follower count seen that year: 79,986.
January
Jan 5 20:00–24:00 UTC Jan 6 00:00–04:00 UTC Jan 8 20:00–24:00 UTC Jan 9 00:00–04:00, 21:00–24:00 UTC Jan 10 00:00–04:00, 20:00–24:00 UTC Jan 11 01:00–04:00, 20:00–24:00 UTC Jan 12 01:00–04:00, 20:00–24:00 UTC Jan 13 01:00–04:00, 20:00–24:00 UTC Jan 14 01:00–04:00, 20:00–24:00 UTC Jan 15 01:00–04:00, 20:00–24:00 UTC Jan 16 01:00–04:00 UTC Jan 17 20:00–24:00 UTC Jan 18 01:00–04:00, 20:00–23:00 UTC Jan 19 01:00–04:00, 21:00–24:00 UTC Jan 20 01:00–04:00, 20:00–24:00 UTC Jan 21 01:00–04:00 UTC Jan 22 20:00–24:00 UTC Jan 23 00:00–04:00, 21:00–24:00 UTC Jan 24 01:00–04:00, 20:00–24:00 UTC Jan 25 01:00–04:00, 22:00–24:00 UTC Jan 26 00:00–04:00, 20:00–24:00 UTC Jan 27 01:00–04:00, 20:00–24:00 UTC Jan 28 01:00–04:00 UTC Jan 30 20:00–24:00 UTC Jan 31 01:00–04:00, 20:00–24:00 UTC
February
Feb 1 02:00–04:00, 20:00–24:00 UTC Feb 2 01:00–04:00, 21:00–24:00 UTC Feb 3 01:00–04:00, 20:00–24:00 UTC Feb 4 00:00–04:00, 20:00–24:00 UTC Feb 5 01:00–04:00 UTC Feb 7 20:00–24:00 UTC Feb 8 01:00–04:00, 21:00–24:00 UTC Feb 9 00:00–04:00, 21:00–24:00 UTC Feb 10 01:00–04:00, 20:00–24:00 UTC Feb 11 00:00–04:00 UTC Feb 12 21:00–24:00 UTC Feb 13 00:00–04:00, 21:00–24:00 UTC Feb 14 02:00–04:00 UTC Feb 15 20:00–24:00 UTC Feb 16 01:00–04:00, 20:00–24:00 UTC Feb 17 01:00–04:00, 20:00–24:00 UTC Feb 18 01:00–04:00 UTC Feb 20 20:00–24:00 UTC Feb 21 01:00–04:00, 20:00–24:00 UTC Feb 22 00:00–04:00, 20:00–24:00 UTC Feb 23 01:00–04:00, 20:00–24:00 UTC Feb 24 01:00–04:00 UTC Feb 26 21:00–24:00 UTC Feb 27 01:00–04:00, 20:00–24:00 UTC Feb 28 01:00–04:00, 20:00–24:00 UTC
March
Mar 1 01:00–04:00, 20:00–24:00 UTC Mar 2 01:00–04:00, 20:00–24:00 UTC Mar 3 00:00–04:00, 20:00–24:00 UTC Mar 4 00:00–05:00, 20:00–24:00 UTC Mar 5 01:00–04:00 UTC Mar 7 21:00–24:00 UTC Mar 8 00:00–04:00, 20:00–24:00 UTC Mar 9 00:00–04:00, 20:00–24:00 UTC Mar 10 01:00–04:00 UTC Mar 12 20:00–24:00 UTC Mar 13 01:00–04:00, 20:00–24:00 UTC Mar 14 01:00–04:00, 20:00–24:00 UTC Mar 15 01:00–04:00, 21:00–24:00 UTC Mar 16 00:00–04:00, 20:00–24:00 UTC Mar 17 00:00–04:00, 20:00–24:00 UTC Mar 18 01:00–04:00 UTC Mar 20 20:00–24:00 UTC Mar 21 01:00–04:00, 20:00–24:00 UTC Mar 22 00:00–04:00, 20:00–24:00 UTC Mar 23 00:00–04:00, 20:00–24:00 UTC Mar 24 00:00–04:00, 20:00–24:00 UTC Mar 25 00:00–04:00, 20:00–24:00 UTC Mar 26 01:00–04:00, 20:00–24:00 UTC Mar 27 01:00–04:00 UTC Mar 29 19:00–23:00 UTC Mar 30 00:00–03:00, 20:00–22:00 UTC Mar 31 00:00–03:00, 19:00–23:00 UTC
April
Apr 1 00:00–03:00, 19:00–23:00 UTC Apr 2 00:00–03:00, 19:00–23:00 UTC Apr 3 00:00–03:00 UTC Apr 4 01:00–03:00, 19:00–23:00 UTC Apr 5 00:00–03:00 UTC Apr 6 19:00–23:00 UTC Apr 7 00:00–03:00, 19:00–23:00 UTC Apr 8 00:00–03:00, 19:00–23:00 UTC Apr 9 00:00–03:00, 22:00–23:00 UTC Apr 10 00:00–03:00 UTC Apr 15 19:00–23:00 UTC Apr 16 00:00–03:00, 19:00–23:00 UTC Apr 17 00:00–03:00, 19:00–23:00 UTC Apr 18 00:00–03:00, 19:00–23:00 UTC Apr 19 00:00–03:00, 19:00–24:00 UTC Apr 20 00:00–03:00, 19:00–23:00 UTC Apr 21 00:00–03:00, 20:00–23:00 UTC Apr 22 00:00–03:00, 19:00–23:00 UTC Apr 23 00:00–03:00, 20:00–23:00 UTC Apr 24 00:00–03:00 UTC Apr 26 19:00–23:00 UTC Apr 27 00:00–03:00, 19:00–23:00 UTC Apr 28 00:00–03:00, 19:00–23:00 UTC Apr 29 00:00–03:00, 19:00–23:00 UTC Apr 30 00:00–03:00 UTC
May
May 3 19:00–24:00 UTC May 4 00:00–03:00, 20:00–23:00 UTC May 7 20:00–22:00 UTC May 8 01:00–03:00, 19:00–24:00 UTC May 9 01:00–03:00, 19:00–23:00 UTC May 10 00:00–03:00, 19:00–23:00 UTC May 11 00:00–03:00, 19:00–23:00 UTC May 12 00:00–03:00, 19:00–23:00 UTC May 13 00:00–03:00, 19:00–23:00 UTC May 14 00:00–01:00, 19:00–20:00, 22:00–23:00 UTC May 15 00:00–02:00, 19:00–24:00 UTC May 17 21:00–23:00 UTC May 18 00:00–03:00, 19:00–24:00 UTC May 19 00:00–03:00, 19:00–23:00 UTC May 20 00:00–02:00 UTC May 22 19:00–24:00 UTC May 23 00:00–03:00, 19:00–23:00 UTC May 24 00:00–03:00, 19:00–24:00 UTC May 25 00:00–03:00, 19:00–23:00 UTC May 26 00:00–03:00, 19:00–23:00 UTC May 27 00:00–03:00, 19:00–23:00 UTC May 28 00:00–03:00, 19:00–24:00 UTC May 29 00:00–03:00, 19:00–23:00 UTC May 30 00:00–03:00 UTC May 31 19:00–23:00 UTC
June
Jun 1 00:00–03:00, 20:00–23:00 UTC Jun 2 00:00–03:00, 19:00–23:00 UTC Jun 3 00:00–03:00, 19:00–24:00 UTC Jun 4 00:00–04:00, 20:00–24:00 UTC Jun 5 00:00–04:00 UTC Jun 7 20:00–24:00 UTC Jun 8 00:00–01:00, 20:00–24:00 UTC Jun 9 00:00–03:00, 19:00–22:00 UTC Jun 10 00:00–02:00, 20:00–23:00 UTC Jun 11 00:00–03:00, 19:00–24:00 UTC Jun 12 00:00–03:00, 20:00–24:00 UTC Jun 13 01:00–04:00 UTC Jun 15 19:00–23:00 UTC Jun 16 00:00–03:00, 19:00–24:00 UTC Jun 17 01:00–03:00 UTC Jun 19 20:00–23:00 UTC Jun 20 00:00–03:00, 19:00–24:00 UTC Jun 21 00:00–03:00, 19:00–22:00 UTC Jun 22 00:00–03:00, 19:00–23:00 UTC Jun 23 19:00–24:00 UTC Jun 24 01:00–02:00, 19:00–23:00 UTC Jun 25 00:00–03:00, 19:00–23:00 UTC Jun 26 00:00–03:00, 19:00–23:00 UTC Jun 27 00:00–03:00 UTC Jun 29 19:00–24:00 UTC Jun 30 00:00–02:00, 19:00–22:00 UTC
July
Jul 1 21:00–23:00 UTC Jul 2 00:00–03:00, 19:00–22:00 UTC Jul 3 00:00–03:00, 19:00–23:00 UTC Jul 4 00:00–03:00 UTC Jul 6 20:00–23:00 UTC Jul 7 00:00–03:00, 19:00–23:00 UTC Jul 8 00:00–03:00, 19:00–24:00 UTC Jul 9 00:00–03:00 UTC Jul 11 19:00–21:00 UTC Jul 12 19:00–21:00 UTC Jul 13 00:00–03:00, 19:00–24:00 UTC Jul 14 00:00–03:00 UTC Jul 15 01:00–03:00 UTC Jul 16 19:00–23:00 UTC Jul 17 00:00–03:00, 20:00–23:00 UTC Jul 18 00:00–03:00 UTC Jul 20 21:00–24:00 UTC Jul 21 00:00–03:00, 19:00–23:00 UTC Jul 22 00:00–03:00 UTC Jul 24 00:00–01:00, 03:00–04:00, 19:00–24:00 UTC Jul 25 00:00–03:00, 19:00–23:00 UTC Jul 26 02:00–03:00 UTC Jul 27 19:00–21:00 UTC