Most often in January, 25 days.
Highest follower count seen that year: 32,401.
January
Jan 3 15:00–23:00 UTC Jan 4 17:00–24:00 UTC Jan 5 00:00–01:00, 18:00–24:00 UTC Jan 8 17:00–23:00 UTC Jan 9 18:00–24:00 UTC Jan 10 16:00–22:00 UTC Jan 11 18:00–24:00 UTC Jan 12 00:00–01:00, 11:00–15:00 UTC Jan 13 15:00–22:00 UTC Jan 14 16:00–21:00 UTC Jan 15 19:00–24:00 UTC Jan 16 00:00–01:00 UTC Jan 17 17:00–22:00 UTC Jan 19 20:00–24:00 UTC Jan 20 00:00–02:00, 17:00–24:00 UTC Jan 21 00:00–01:00, 18:00–24:00 UTC Jan 22 00:00–01:00, 18:00–24:00 UTC Jan 23 00:00–01:00, 18:00–24:00 UTC Jan 24 01:00–02:00, 16:00–22:00 UTC Jan 26 17:00–24:00 UTC Jan 27 16:00–20:00 UTC Jan 28 17:00–24:00 UTC Jan 29 00:00–01:00, 17:00–24:00 UTC Jan 30 17:00–24:00 UTC Jan 31 00:00–01:00, 16:00–22:00 UTC
February
Feb 3 19:00–24:00 UTC Feb 4 01:00–02:00, 19:00–24:00 UTC Feb 5 00:00–02:00, 13:00–20:00 UTC Feb 6 15:00–22:00 UTC Feb 7 16:00–22:00 UTC Feb 10 15:00–24:00 UTC Feb 11 18:00–24:00 UTC Feb 12 00:00–02:00, 16:00–24:00 UTC Feb 13 18:00–24:00 UTC Feb 14 00:00–03:00 UTC Feb 16 15:00–23:00 UTC Feb 17 18:00–24:00 UTC Feb 18 00:00–01:00, 19:00–24:00 UTC Feb 19 00:00–01:00, 21:00–24:00 UTC Feb 20 00:00–01:00 UTC Feb 21 18:00–23:00 UTC Feb 23 16:00–24:00 UTC Feb 24 18:00–24:00 UTC Feb 26 14:00–22:00 UTC Feb 27 16:00–21:00 UTC Feb 28 18:00–23:00 UTC
March
Mar 2 19:00–24:00 UTC Mar 3 00:00–02:00, 19:00–23:00 UTC Mar 4 00:00–03:00, 18:00–23:00 UTC Mar 5 01:00–02:00, 18:00–24:00 UTC Mar 6 00:00–01:00 UTC Mar 9 21:00–24:00 UTC Mar 10 00:00–03:00, 21:00–24:00 UTC Mar 11 00:00–04:00, 11:00–23:00 UTC Mar 12 19:00–24:00 UTC Mar 13 00:00–03:00, 18:00–24:00 UTC Mar 14 00:00–02:00, 18:00–23:00 UTC Mar 15 00:00–01:00, 17:00–22:00 UTC Mar 17 19:00–23:00 UTC Mar 19 17:00–24:00 UTC Mar 20 00:00–02:00, 19:00–24:00 UTC Mar 21 00:00–01:00, 19:00–24:00 UTC Mar 22 00:00–02:00, 19:00–24:00 UTC Mar 23 00:00–04:00, 21:00–24:00 UTC Mar 24 00:00–04:00, 18:00–24:00 UTC Mar 25 19:00–24:00 UTC Mar 26 00:00–02:00, 18:00–24:00 UTC Mar 27 19:00–24:00 UTC Mar 28 00:00–03:00, 18:00–24:00 UTC Mar 29 00:00–01:00 UTC
April
Apr 4 16:00–24:00 UTC Apr 6 19:00–24:00 UTC Apr 7 00:00–01:00, 16:00–24:00 UTC Apr 8 00:00–01:00, 13:00–18:00 UTC Apr 9 22:00–24:00 UTC Apr 10 00:00–02:00, 20:00–21:00 UTC Apr 14 18:00–24:00 UTC Apr 15 00:00–01:00, 19:00–24:00 UTC Apr 16 00:00–01:00, 20:00–24:00 UTC Apr 17 18:00–24:00 UTC Apr 18 18:00–24:00 UTC Apr 20 17:00–22:00 UTC Apr 21 17:00–23:00 UTC Apr 22 19:00–24:00 UTC Apr 23 00:00–01:00, 18:00–24:00 UTC Apr 24 18:00–23:00 UTC Apr 25 15:00–24:00 UTC Apr 27 17:00–24:00 UTC Apr 28 18:00–24:00 UTC Apr 29 19:00–24:00 UTC
May
May 1 16:00–24:00 UTC May 4 19:00–24:00 UTC May 5 17:00–22:00 UTC May 6 19:00–24:00 UTC May 7 00:00–01:00, 17:00–21:00 UTC May 8 17:00–24:00 UTC May 9 18:00–24:00 UTC May 11 18:00–21:00 UTC May 12 17:00–24:00 UTC May 14 18:00–23:00 UTC May 15 18:00–24:00 UTC May 16 15:00–20:00 UTC May 20 19:00–22:00 UTC May 21 17:00–24:00 UTC May 22 18:00–24:00 UTC May 23 00:00–02:00, 21:00–22:00 UTC May 25 17:00–24:00 UTC May 26 00:00–01:00, 18:00–23:00 UTC May 27 20:00–24:00 UTC May 28 00:00–02:00, 17:00–24:00 UTC May 29 00:00–01:00, 15:00–21:00 UTC May 30 15:00–22:00 UTC
June
Jun 1 17:00–23:00 UTC Jun 3 18:00–24:00 UTC Jun 4 20:00–24:00 UTC Jun 5 00:00–02:00, 17:00–24:00 UTC Jun 6 00:00–01:00, 18:00–24:00 UTC Jun 7 00:00–02:00 UTC Jun 8 18:00–24:00 UTC Jun 9 18:00–24:00 UTC Jun 10 17:00–24:00 UTC Jun 11 19:00–24:00 UTC Jun 12 00:00–01:00 UTC Jun 13 18:00–19:00 UTC Jun 15 17:00–22:00 UTC Jun 18 18:00–24:00 UTC Jun 19 00:00–01:00, 18:00–24:00 UTC Jun 20 00:00–01:00 UTC Jun 24 16:00–22:00 UTC Jun 25 18:00–24:00 UTC Jun 26 00:00–01:00, 17:00–23:00 UTC Jun 27 12:00–17:00 UTC Jun 28 17:00–24:00 UTC Jun 29 18:00–24:00 UTC Jun 30 00:00–01:00, 16:00–24:00 UTC
July
Jul 1 16:00–24:00 UTC Jul 2 18:00–24:00 UTC Jul 3 17:00–24:00 UTC Jul 4 18:00–24:00 UTC Jul 5 00:00–01:00 UTC Jul 8 18:00–24:00 UTC Jul 9 00:00–01:00, 18:00–24:00 UTC Jul 10 17:00–24:00 UTC Jul 11 01:00–02:00, 17:00–24:00 UTC Jul 12 00:00–01:00, 19:00–24:00 UTC Jul 13 00:00–01:00, 19:00–24:00 UTC Jul 14 00:00–01:00, 19:00–24:00 UTC Jul 15 18:00–24:00 UTC Jul 16 00:00–02:00 UTC Jul 18 18:00–24:00 UTC Jul 19 00:00–01:00, 17:00–24:00 UTC Jul 21 00:00–03:00 UTC Jul 22 19:00–24:00 UTC Jul 23 00:00–01:00, 20:00–24:00 UTC Jul 24 00:00–02:00 UTC Jul 25 18:00–24:00 UTC Jul 26 17:00–24:00 UTC Jul 27 19:00–21:00 UTC