Most often in March, 23 days.
Highest follower count seen that year: 57,801.
January
Jan 7 17:00–24:00 UTC Jan 8 00:00–01:00, 17:00–24:00 UTC Jan 9 00:00–01:00, 18:00–24:00 UTC Jan 10 00:00–01:00, 19:00–23:00 UTC Jan 11 18:00–22:00 UTC Jan 12 15:00–21:00 UTC Jan 13 17:00–24:00 UTC Jan 14 00:00–01:00, 17:00–24:00 UTC Jan 15 17:00–24:00 UTC Jan 16 17:00–24:00 UTC Jan 17 00:00–01:00, 17:00–22:00 UTC Jan 19 17:00–24:00 UTC Jan 20 00:00–01:00, 16:00–24:00 UTC Jan 21 00:00–01:00, 17:00–24:00 UTC Jan 22 17:00–24:00 UTC Jan 23 00:00–01:00, 17:00–24:00 UTC Jan 24 17:00–24:00 UTC Jan 25 17:00–20:00 UTC Jan 26 16:00–24:00 UTC Jan 27 00:00–02:00, 16:00–24:00 UTC Jan 28 00:00–02:00, 17:00–24:00 UTC Jan 29 00:00–01:00, 17:00–24:00 UTC Jan 30 00:00–01:00, 17:00–24:00 UTC
February
Feb 1 03:00–07:00, 11:00–14:00 UTC Feb 2 01:00–06:00, 09:00–13:00 UTC Feb 3 01:00–07:00, 10:00–13:00 UTC Feb 4 01:00–04:00, 09:00–13:00 UTC Feb 5 00:00–06:00, 10:00–13:00 UTC Feb 6 01:00–07:00, 09:00–13:00 UTC Feb 9 00:00–07:00, 09:00–14:00 UTC Feb 10 00:00–13:00 UTC Feb 11 01:00–13:00 UTC Feb 12 01:00–07:00, 09:00–13:00 UTC Feb 13 01:00–07:00, 09:00–13:00 UTC Feb 15 01:00–13:00 UTC Feb 16 00:00–07:00, 09:00–13:00 UTC Feb 17 00:00–07:00, 09:00–13:00 UTC Feb 18 01:00–06:00, 11:00–13:00 UTC Feb 19 01:00–07:00, 09:00–13:00 UTC Feb 20 01:00–07:00, 10:00–13:00 UTC Feb 23 00:00–08:00, 10:00–13:00 UTC Feb 24 00:00–07:00, 09:00–13:00 UTC Feb 25 00:00–07:00, 09:00–13:00 UTC Feb 26 01:00–07:00, 09:00–14:00 UTC Feb 27 01:00–06:00, 09:00–15:00 UTC
March
Mar 1 02:00–13:00 UTC Mar 2 00:00–13:00 UTC Mar 3 00:00–07:00, 09:00–14:00 UTC Mar 4 01:00–13:00 UTC Mar 5 00:00–14:00 UTC Mar 6 00:00–07:00, 10:00–14:00 UTC Mar 9 00:00–13:00, 22:00–24:00 UTC Mar 10 00:00–07:00, 10:00–13:00 UTC Mar 11 01:00–07:00, 10:00–13:00 UTC Mar 12 01:00–07:00, 09:00–13:00 UTC Mar 13 01:00–13:00 UTC Mar 15 01:00–07:00, 09:00–12:00 UTC Mar 16 00:00–14:00 UTC Mar 17 00:00–07:00, 09:00–14:00 UTC Mar 18 02:00–07:00, 10:00–13:00 UTC Mar 19 01:00–07:00, 09:00–13:00 UTC Mar 20 00:00–03:00, 05:00–07:00, 10:00–15:00 UTC Mar 23 00:00–07:00, 09:00–13:00 UTC Mar 24 01:00–07:00, 09:00–13:00 UTC Mar 25 01:00–07:00, 09:00–13:00 UTC Mar 26 02:00–07:00, 10:00–13:00 UTC Mar 27 01:00–07:00, 09:00–13:00 UTC Mar 31 01:00–07:00, 10:00–13:00 UTC
April
Apr 1 01:00–07:00, 09:00–15:00 UTC Apr 2 01:00–07:00, 10:00–14:00 UTC Apr 3 01:00–08:00, 10:00–14:00 UTC Apr 6 01:00–07:00, 10:00–14:00 UTC Apr 7 00:00–07:00, 10:00–13:00 UTC Apr 8 00:00–07:00, 10:00–14:00 UTC Apr 9 01:00–07:00, 10:00–14:00 UTC Apr 10 01:00–07:00, 10:00–14:00 UTC Apr 13 01:00–07:00, 10:00–14:00 UTC Apr 14 01:00–07:00, 10:00–13:00 UTC Apr 15 02:00–07:00, 10:00–13:00 UTC Apr 16 01:00–07:00, 10:00–14:00 UTC Apr 20 01:00–07:00, 10:00–14:00 UTC Apr 21 01:00–07:00, 10:00–15:00 UTC Apr 22 01:00–07:00, 10:00–14:00 UTC Apr 24 03:00–08:00, 10:00–15:00 UTC Apr 27 01:00–07:00, 10:00–13:00 UTC Apr 28 01:00–07:00, 10:00–14:00 UTC Apr 29 01:00–07:00, 10:00–15:00 UTC Apr 30 02:00–07:00, 10:00–14:00 UTC
May
May 1 01:00–06:00, 13:00–15:00 UTC May 4 05:00–14:00 UTC May 5 01:00–06:00, 10:00–14:00 UTC May 6 01:00–07:00, 10:00–14:00 UTC May 7 01:00–07:00, 10:00–13:00 UTC May 8 05:00–07:00, 10:00–14:00 UTC May 11 02:00–07:00, 10:00–14:00 UTC May 12 01:00–06:00, 10:00–13:00 UTC May 13 01:00–05:00, 10:00–14:00 UTC May 14 04:00–07:00, 10:00–12:00 UTC May 15 02:00–07:00, 09:00–14:00 UTC May 18 02:00–07:00, 10:00–15:00 UTC May 19 01:00–07:00, 10:00–14:00 UTC May 20 01:00–07:00, 10:00–15:00 UTC May 21 01:00–07:00, 10:00–13:00 UTC May 22 01:00–07:00, 10:00–14:00 UTC May 25 02:00–07:00, 10:00–14:00 UTC May 26 01:00–07:00, 10:00–13:00 UTC May 27 01:00–07:00, 10:00–13:00 UTC May 28 01:00–09:00 UTC May 29 01:00–07:00, 10:00–11:00 UTC
June
Jun 1 02:00–07:00, 10:00–14:00 UTC Jun 2 01:00–07:00, 10:00–14:00 UTC Jun 3 01:00–08:00, 10:00–14:00 UTC Jun 4 02:00–08:00, 10:00–15:00 UTC Jun 8 03:00–07:00, 10:00–11:00 UTC Jun 9 01:00–07:00, 11:00–14:00 UTC Jun 10 02:00–07:00, 11:00–15:00 UTC Jun 11 01:00–08:00, 10:00–16:00 UTC Jun 12 01:00–08:00, 11:00–15:00 UTC Jun 15 03:00–07:00, 10:00–15:00 UTC Jun 16 01:00–08:00, 10:00–14:00 UTC Jun 17 04:00–07:00, 10:00–14:00 UTC Jun 18 01:00–07:00, 10:00–15:00 UTC Jun 19 01:00–07:00, 10:00–12:00 UTC Jun 23 04:00–07:00, 10:00–14:00 UTC Jun 24 02:00–08:00, 10:00–14:00 UTC Jun 25 02:00–07:00, 10:00–14:00 UTC Jun 26 01:00–07:00, 10:00–14:00 UTC Jun 29 02:00–07:00, 10:00–14:00 UTC Jun 30 02:00–07:00, 10:00–12:00 UTC
July
Jul 12 05:00–10:00 UTC Jul 13 05:00–08:00 UTC Jul 15 15:00–22:00 UTC Jul 16 13:00–20:00 UTC Jul 19 06:00–08:00 UTC Jul 20 06:00–08:00 UTC Jul 21 13:00–21:00 UTC Jul 22 14:00–22:00 UTC Jul 23 17:00–23:00 UTC Jul 24 05:00–08:00 UTC Jul 26 09:00–11:00 UTC Jul 27 06:00–07:00, 20:00–21:00 UTC