Most often in March, 31 days.
Highest follower count seen that year: 3,907.
January
Jan 16 09:00–14:00 UTC Jan 17 04:00–09:00 UTC Jan 22 12:00–20:00 UTC Jan 23 12:00–19:00 UTC Jan 24 12:00–19:00 UTC Jan 25 05:00–15:00 UTC Jan 26 05:00–15:00 UTC Jan 27 05:00–15:00 UTC Jan 28 09:00–18:00 UTC Jan 29 04:00–15:00 UTC Jan 30 05:00–17:00 UTC Jan 31 04:00–07:00, 09:00–13:00 UTC
February
Feb 1 06:00–15:00 UTC Feb 2 06:00–17:00 UTC Feb 3 05:00–16:00 UTC Feb 4 04:00–05:00, 09:00–16:00 UTC Feb 5 05:00–08:00, 15:00–19:00 UTC Feb 6 04:00–16:00 UTC Feb 7 05:00–13:00, 15:00–17:00 UTC Feb 8 04:00–15:00 UTC Feb 9 05:00–16:00 UTC Feb 10 04:00–15:00 UTC Feb 11 04:00–08:00, 10:00–16:00 UTC Feb 12 04:00–08:00, 10:00–16:00 UTC Feb 13 05:00–16:00 UTC Feb 14 04:00–09:00, 11:00–15:00 UTC Feb 15 04:00–16:00, 18:00–24:00 UTC Feb 16 00:00–01:00, 05:00–16:00 UTC Feb 17 04:00–15:00 UTC Feb 19 04:00–16:00 UTC Feb 20 05:00–15:00 UTC Feb 21 04:00–15:00 UTC Feb 22 04:00–16:00 UTC Feb 23 04:00–09:00, 11:00–16:00 UTC Feb 24 11:00–16:00 UTC Feb 25 04:00–16:00 UTC Feb 26 04:00–16:00 UTC Feb 27 05:00–08:00, 10:00–17:00 UTC Feb 28 04:00–09:00 UTC
March
Mar 1 06:00–17:00 UTC Mar 2 04:00–08:00 UTC Mar 3 06:00–16:00 UTC Mar 4 04:00–08:00, 10:00–20:00 UTC Mar 5 04:00–08:00, 13:00–17:00 UTC Mar 6 08:00–10:00, 12:00–16:00 UTC Mar 7 05:00–08:00, 11:00–16:00 UTC Mar 8 05:00–16:00 UTC Mar 9 04:00–16:00 UTC Mar 10 05:00–21:00 UTC Mar 11 04:00–08:00, 10:00–16:00, 18:00–21:00 UTC Mar 12 04:00–08:00, 11:00–15:00, 18:00–20:00 UTC Mar 13 05:00–09:00, 11:00–16:00, 18:00–20:00 UTC Mar 14 04:00–15:00, 17:00–21:00 UTC Mar 15 05:00–11:00 UTC Mar 16 04:00–10:00, 12:00–15:00 UTC Mar 17 03:00–14:00, 18:00–20:00 UTC Mar 18 05:00–09:00, 12:00–14:00 UTC Mar 19 04:00–09:00 UTC Mar 20 05:00–12:00 UTC Mar 21 09:00–17:00 UTC Mar 22 04:00–09:00, 11:00–12:00, 17:00–18:00 UTC Mar 23 04:00–08:00, 11:00–15:00, 17:00–19:00 UTC Mar 24 04:00–15:00 UTC Mar 25 04:00–08:00, 10:00–11:00, 13:00–16:00 UTC Mar 26 05:00–11:00, 13:00–15:00 UTC Mar 27 03:00–08:00, 11:00–12:00, 18:00–19:00 UTC Mar 28 02:00–08:00, 10:00–16:00 UTC Mar 29 12:00–13:00, 19:00–21:00 UTC Mar 30 06:00–16:00 UTC Mar 31 05:00–13:00 UTC
April
Apr 1 06:00–11:00 UTC Apr 2 05:00–08:00 UTC Apr 3 07:00–15:00 UTC Apr 4 04:00–09:00, 12:00–15:00 UTC Apr 5 05:00–09:00, 11:00–14:00 UTC Apr 6 04:00–09:00, 11:00–14:00, 18:00–20:00 UTC Apr 7 04:00–08:00, 10:00–13:00, 17:00–18:00 UTC Apr 8 05:00–08:00, 11:00–15:00 UTC Apr 9 04:00–09:00, 13:00–15:00 UTC Apr 10 17:00–19:00 UTC Apr 11 06:00–09:00, 11:00–15:00 UTC Apr 12 04:00–09:00, 11:00–15:00 UTC Apr 13 05:00–10:00, 12:00–14:00 UTC Apr 14 05:00–10:00, 12:00–13:00 UTC Apr 15 05:00–08:00 UTC Apr 16 12:00–13:00 UTC Apr 17 10:00–16:00 UTC Apr 18 05:00–09:00, 11:00–15:00 UTC Apr 19 05:00–08:00, 12:00–16:00 UTC Apr 20 04:00–09:00, 11:00–15:00 UTC Apr 21 04:00–08:00, 10:00–13:00 UTC Apr 22 04:00–16:00 UTC Apr 23 05:00–06:00, 10:00–16:00 UTC Apr 24 05:00–08:00, 10:00–16:00 UTC Apr 25 05:00–08:00, 11:00–16:00 UTC Apr 26 05:00–08:00, 10:00–15:00 UTC Apr 27 04:00–16:00 UTC Apr 28 05:00–09:00, 11:00–16:00 UTC Apr 29 05:00–10:00, 12:00–16:00 UTC Apr 30 05:00–09:00 UTC
May
May 1 05:00–08:00, 13:00–16:00 UTC May 2 05:00–14:00 UTC May 3 06:00–08:00, 11:00–16:00 UTC May 4 05:00–10:00, 12:00–16:00 UTC May 6 05:00–10:00, 12:00–13:00 UTC May 7 06:00–10:00, 12:00–16:00 UTC May 8 12:00–15:00 UTC May 9 05:00–11:00, 14:00–16:00 UTC May 10 04:00–15:00 UTC May 11 05:00–15:00 UTC May 12 04:00–12:00, 14:00–16:00 UTC May 13 05:00–15:00 UTC May 14 05:00–08:00, 10:00–16:00 UTC May 15 05:00–10:00 UTC May 16 05:00–12:00, 14:00–16:00 UTC May 17 04:00–15:00 UTC May 18 04:00–12:00, 14:00–15:00 UTC May 19 05:00–16:00 UTC May 20 05:00–09:00 UTC May 21 05:00–06:00 UTC May 22 05:00–16:00 UTC May 23 05:00–10:00, 12:00–16:00 UTC May 24 05:00–16:00 UTC May 25 05:00–11:00 UTC May 26 05:00–15:00 UTC