Most often in April, 23 days.
Highest follower count seen that year: 96,951.
January
Jan 8 21:00–24:00 UTC Jan 9 00:00–01:00, 21:00–24:00 UTC Jan 10 00:00–02:00, 19:00–22:00 UTC Jan 12 21:00–24:00 UTC Jan 13 00:00–02:00, 21:00–24:00 UTC Jan 14 00:00–02:00, 21:00–24:00 UTC Jan 15 00:00–01:00, 21:00–23:00 UTC Jan 20 21:00–24:00 UTC Jan 21 00:00–03:00, 21:00–24:00 UTC Jan 22 00:00–02:00, 21:00–24:00 UTC Jan 23 00:00–02:00, 21:00–24:00 UTC Jan 24 00:00–02:00 UTC Jan 26 21:00–24:00 UTC Jan 27 00:00–03:00 UTC Jan 28 21:00–24:00 UTC Jan 29 00:00–02:00, 21:00–24:00 UTC Jan 30 21:00–24:00 UTC Jan 31 00:00–01:00 UTC
February
Feb 3 21:00–24:00 UTC Feb 4 00:00–03:00, 21:00–24:00 UTC Feb 5 00:00–02:00, 21:00–24:00 UTC Feb 6 00:00–03:00, 21:00–24:00 UTC Feb 7 00:00–01:00 UTC Feb 9 21:00–24:00 UTC Feb 10 00:00–02:00, 21:00–24:00 UTC Feb 11 00:00–03:00, 21:00–24:00 UTC Feb 12 00:00–02:00, 21:00–24:00 UTC Feb 13 00:00–02:00, 21:00–24:00 UTC Feb 14 00:00–01:00 UTC Feb 16 21:00–24:00 UTC Feb 17 00:00–03:00, 21:00–24:00 UTC Feb 18 00:00–02:00, 21:00–24:00 UTC Feb 19 00:00–02:00, 21:00–24:00 UTC Feb 20 00:00–02:00, 21:00–24:00 UTC Feb 21 00:00–02:00 UTC Feb 23 21:00–24:00 UTC Feb 24 00:00–03:00, 21:00–24:00 UTC Feb 25 00:00–03:00, 21:00–24:00 UTC Feb 26 00:00–03:00, 21:00–23:00 UTC Feb 27 20:00–24:00 UTC Feb 28 00:00–01:00 UTC
March
Mar 2 21:00–24:00 UTC Mar 3 00:00–03:00, 22:00–24:00 UTC Mar 4 00:00–03:00 UTC Mar 12 21:00–24:00 UTC Mar 13 00:00–02:00, 20:00–24:00 UTC Mar 14 00:00–02:00, 19:00–24:00 UTC Mar 16 21:00–24:00 UTC Mar 17 00:00–04:00, 21:00–24:00 UTC Mar 18 00:00–02:00, 21:00–24:00 UTC Mar 19 00:00–02:00, 21:00–23:00 UTC Mar 20 21:00–24:00 UTC Mar 21 00:00–01:00 UTC Mar 23 21:00–24:00 UTC Mar 24 00:00–03:00, 21:00–24:00 UTC Mar 25 00:00–01:00, 21:00–24:00 UTC Mar 26 00:00–03:00, 21:00–24:00 UTC Mar 27 00:00–02:00, 21:00–24:00 UTC Mar 28 00:00–02:00 UTC Mar 30 21:00–24:00 UTC Mar 31 00:00–02:00, 22:00–24:00 UTC
April
Apr 1 00:00–05:00, 22:00–24:00 UTC Apr 2 00:00–01:00 UTC Apr 6 21:00–24:00 UTC Apr 7 00:00–03:00, 21:00–24:00 UTC Apr 8 00:00–03:00, 21:00–24:00 UTC Apr 9 00:00–02:00, 21:00–24:00 UTC Apr 10 00:00–02:00, 21:00–24:00 UTC Apr 11 00:00–02:00 UTC Apr 13 21:00–24:00 UTC Apr 14 00:00–02:00, 21:00–24:00 UTC Apr 15 00:00–03:00, 21:00–24:00 UTC Apr 16 00:00–02:00, 21:00–24:00 UTC Apr 17 00:00–02:00, 21:00–24:00 UTC Apr 18 00:00–02:00 UTC Apr 20 21:00–24:00 UTC Apr 21 00:00–03:00, 21:00–24:00 UTC Apr 22 00:00–02:00, 21:00–24:00 UTC Apr 23 00:00–02:00, 21:00–24:00 UTC Apr 24 00:00–02:00, 21:00–24:00 UTC Apr 25 00:00–02:00 UTC Apr 28 23:00–24:00 UTC Apr 29 00:00–03:00, 21:00–24:00 UTC Apr 30 00:00–02:00, 21:00–24:00 UTC
May
May 1 00:00–02:00, 12:00–14:00 UTC May 4 21:00–24:00 UTC May 5 00:00–03:00, 21:00–24:00 UTC May 6 00:00–03:00, 21:00–24:00 UTC May 7 00:00–04:00, 21:00–24:00 UTC May 8 00:00–02:00, 21:00–24:00 UTC May 9 00:00–03:00 UTC May 12 21:00–24:00 UTC May 13 00:00–03:00, 21:00–24:00 UTC May 14 00:00–02:00, 21:00–24:00 UTC May 15 00:00–02:00, 21:00–24:00 UTC May 16 00:00–03:00 UTC May 18 21:00–24:00 UTC May 19 00:00–03:00, 22:00–24:00 UTC May 20 00:00–03:00, 21:00–24:00 UTC May 21 00:00–03:00, 21:00–24:00 UTC May 22 00:00–04:00 UTC May 25 21:00–24:00 UTC May 26 00:00–03:00, 21:00–24:00 UTC May 27 00:00–02:00, 21:00–24:00 UTC May 28 00:00–02:00, 21:00–24:00 UTC May 29 00:00–02:00, 20:00–24:00 UTC
June
Jun 1 22:00–24:00 UTC Jun 2 00:00–02:00, 21:00–24:00 UTC Jun 3 00:00–02:00, 21:00–24:00 UTC Jun 4 00:00–02:00, 21:00–24:00 UTC Jun 5 00:00–02:00, 21:00–24:00 UTC Jun 6 00:00–02:00 UTC Jun 8 22:00–24:00 UTC Jun 9 00:00–02:00, 22:00–24:00 UTC Jun 10 00:00–02:00, 21:00–24:00 UTC Jun 11 00:00–02:00, 21:00–24:00 UTC Jun 12 00:00–02:00, 21:00–24:00 UTC Jun 13 00:00–02:00 UTC Jun 17 19:00–24:00 UTC Jun 18 21:00–24:00 UTC Jun 19 00:00–02:00, 21:00–24:00 UTC Jun 20 00:00–02:00, 14:00–18:00 UTC Jun 22 21:00–24:00 UTC Jun 23 00:00–02:00, 20:00–24:00 UTC Jun 24 00:00–01:00, 21:00–24:00 UTC Jun 25 00:00–02:00, 21:00–24:00 UTC Jun 26 00:00–02:00, 14:00–19:00 UTC Jun 27 14:00–19:00 UTC Jun 30 21:00–24:00 UTC
July
Jul 1 00:00–03:00, 21:00–24:00 UTC Jul 2 00:00–02:00, 21:00–24:00 UTC Jul 3 00:00–02:00, 14:00–19:00 UTC Jul 6 21:00–24:00 UTC Jul 7 00:00–02:00, 14:00–18:00 UTC Jul 8 21:00–24:00 UTC Jul 9 00:00–02:00, 14:00–15:00 UTC Jul 11 16:00–20:00 UTC Jul 13 21:00–24:00 UTC Jul 14 00:00–01:00 UTC Jul 15 22:00–24:00 UTC Jul 17 14:00–19:00 UTC Jul 18 14:00–19:00 UTC Jul 20 21:00–24:00 UTC Jul 21 00:00–02:00, 13:00–18:00 UTC Jul 22 21:00–24:00 UTC Jul 23 20:00–24:00 UTC Jul 24 00:00–02:00, 21:00–24:00 UTC Jul 25 00:00–01:00 UTC