Most often in May, 25 days.
Highest follower count seen that year: 47,149.
January
Jan 4 20:00–24:00 UTC Jan 5 00:00–05:00, 12:00–17:00 UTC Jan 6 19:00–22:00 UTC Jan 11 08:00–12:00, 15:00–17:00, 21:00–24:00 UTC Jan 12 14:00–24:00 UTC Jan 13 07:00–09:00, 13:00–15:00, 19:00–23:00 UTC Jan 14 14:00–20:00 UTC Jan 15 08:00–12:00, 15:00–17:00, 19:00–20:00 UTC Jan 19 08:00–12:00, 19:00–21:00 UTC Jan 20 08:00–09:00, 13:00–17:00 UTC Jan 22 20:00–23:00 UTC Jan 23 06:00–11:00, 13:00–16:00 UTC Jan 24 06:00–09:00 UTC Jan 25 22:00–24:00 UTC Jan 26 10:00–14:00 UTC Jan 27 07:00–09:00, 12:00–13:00, 15:00–17:00, 20:00–22:00 UTC Jan 28 16:00–20:00 UTC Jan 29 07:00–08:00 UTC Jan 30 05:00–07:00, 09:00–11:00, 14:00–16:00 UTC Jan 31 07:00–11:00, 13:00–23:00 UTC
February
Feb 1 14:00–16:00, 18:00–20:00 UTC Feb 2 06:00–09:00 UTC Feb 3 08:00–10:00, 13:00–15:00, 19:00–20:00 UTC Feb 4 13:00–14:00 UTC Feb 5 10:00–11:00 UTC Feb 6 06:00–09:00 UTC Feb 9 06:00–12:00 UTC Feb 10 08:00–10:00 UTC Feb 11 07:00–09:00 UTC Feb 12 07:00–10:00, 19:00–22:00 UTC Feb 13 07:00–11:00, 19:00–20:00 UTC Feb 14 06:00–11:00 UTC Feb 15 07:00–16:00, 19:00–22:00 UTC Feb 16 08:00–12:00 UTC Feb 18 07:00–10:00 UTC Feb 19 08:00–10:00 UTC Feb 20 07:00–09:00, 14:00–17:00 UTC Feb 21 12:00–16:00 UTC Feb 22 07:00–10:00 UTC Feb 24 07:00–11:00 UTC Feb 25 08:00–11:00, 19:00–21:00 UTC Feb 27 07:00–11:00, 19:00–21:00 UTC Feb 28 07:00–14:00, 19:00–23:00 UTC
March
Mar 1 08:00–10:00 UTC Mar 3 12:00–15:00 UTC Mar 4 11:00–14:00 UTC Mar 5 09:00–11:00 UTC Mar 6 08:00–12:00 UTC Mar 7 09:00–13:00, 17:00–19:00 UTC Mar 8 09:00–18:00 UTC Mar 9 15:00–17:00 UTC Mar 11 19:00–21:00 UTC Mar 12 06:00–12:00 UTC Mar 13 07:00–11:00 UTC Mar 14 09:00–15:00, 18:00–20:00 UTC Mar 15 08:00–16:00 UTC Mar 16 06:00–10:00 UTC Mar 17 08:00–10:00 UTC Mar 18 07:00–09:00 UTC Mar 19 06:00–09:00 UTC Mar 22 11:00–12:00, 16:00–20:00 UTC Mar 23 07:00–12:00 UTC Mar 24 06:00–12:00 UTC Mar 26 15:00–18:00, 20:00–22:00 UTC Mar 27 06:00–15:00 UTC Mar 28 07:00–19:00 UTC Mar 29 07:00–11:00 UTC
April
Apr 2 12:00–15:00 UTC Apr 3 06:00–08:00 UTC Apr 4 10:00–13:00 UTC Apr 5 09:00–11:00, 14:00–17:00 UTC Apr 6 14:00–17:00 UTC Apr 7 06:00–12:00 UTC Apr 10 07:00–15:00 UTC Apr 11 07:00–13:00 UTC Apr 12 09:00–11:00 UTC Apr 13 09:00–16:00 UTC Apr 14 05:00–06:00, 15:00–17:00 UTC Apr 15 10:00–13:00 UTC Apr 17 08:00–09:00 UTC Apr 18 08:00–15:00 UTC Apr 19 06:00–11:00 UTC Apr 23 09:00–11:00 UTC Apr 24 09:00–12:00, 14:00–15:00 UTC Apr 25 06:00–11:00 UTC Apr 26 08:00–13:00, 16:00–18:00 UTC Apr 27 08:00–12:00, 17:00–18:00 UTC Apr 28 07:00–11:00 UTC Apr 30 20:00–22:00 UTC
May
May 1 07:00–10:00 UTC May 2 09:00–14:00 UTC May 3 12:00–14:00, 16:00–19:00 UTC May 4 07:00–12:00 UTC May 5 07:00–09:00, 11:00–14:00 UTC May 8 06:00–11:00 UTC May 9 07:00–12:00, 14:00–15:00 UTC May 12 08:00–17:00 UTC May 13 07:00–12:00 UTC May 14 07:00–09:00, 19:00–20:00 UTC May 15 06:00–09:00, 16:00–17:00 UTC May 16 07:00–14:00 UTC May 17 08:00–13:00 UTC May 18 06:00–12:00 UTC May 19 06:00–10:00 UTC May 20 06:00–10:00 UTC May 22 08:00–11:00 UTC May 23 07:00–15:00 UTC May 24 20:00–22:00 UTC May 25 07:00–10:00, 13:00–17:00 UTC May 26 08:00–11:00, 19:00–21:00 UTC May 28 10:00–13:00, 15:00–17:00, 20:00–21:00 UTC May 29 06:00–08:00 UTC May 30 08:00–12:00, 15:00–17:00 UTC May 31 09:00–16:00 UTC
June
Jun 1 10:00–15:00 UTC Jun 4 09:00–12:00 UTC Jun 5 05:00–09:00 UTC Jun 6 09:00–13:00 UTC Jun 8 08:00–12:00 UTC Jun 9 08:00–15:00 UTC Jun 10 08:00–13:00 UTC Jun 11 07:00–10:00 UTC Jun 12 07:00–09:00 UTC Jun 13 10:00–15:00 UTC Jun 15 08:00–13:00 UTC Jun 16 15:00–18:00 UTC Jun 17 09:00–12:00 UTC Jun 19 08:00–10:00, 15:00–17:00 UTC Jun 20 10:00–12:00, 16:00–22:00 UTC Jun 22 10:00–15:00 UTC Jun 23 19:00–21:00 UTC Jun 24 07:00–12:00 UTC Jun 25 19:00–20:00 UTC Jun 26 06:00–13:00 UTC Jun 28 09:00–15:00, 17:00–18:00 UTC Jun 29 09:00–12:00 UTC Jun 30 06:00–09:00 UTC
July
Jul 3 13:00–15:00 UTC Jul 4 10:00–16:00 UTC Jul 5 11:00–15:00, 20:00–21:00 UTC Jul 6 08:00–12:00, 18:00–20:00 UTC Jul 7 08:00–13:00, 18:00–20:00 UTC Jul 8 10:00–15:00 UTC Jul 9 07:00–13:00, 20:00–21:00 UTC Jul 10 06:00–09:00 UTC Jul 13 13:00–19:00 UTC Jul 14 07:00–10:00 UTC Jul 18 10:00–16:00 UTC Jul 19 08:00–15:00 UTC Jul 20 12:00–17:00 UTC Jul 21 07:00–12:00 UTC Jul 24 09:00–11:00, 15:00–18:00 UTC Jul 26 11:00–14:00 UTC