Most often in May, 28 days.
Highest follower count seen that year: 334,820.
January
Jan 2 08:00–16:00 UTC Jan 3 08:00–17:00 UTC Jan 4 08:00–16:00 UTC Jan 5 08:00–17:00 UTC Jan 9 09:00–18:00 UTC Jan 10 06:00–15:00 UTC Jan 11 07:00–16:00 UTC Jan 12 09:00–18:00 UTC Jan 14 09:00–18:00 UTC Jan 16 09:00–18:00 UTC Jan 17 08:00–18:00 UTC Jan 18 09:00–18:00 UTC Jan 19 08:00–17:00 UTC Jan 21 07:00–17:00 UTC Jan 24 08:00–17:00 UTC Jan 25 08:00–18:00 UTC Jan 26 07:00–16:00 UTC Jan 28 10:00–18:00 UTC Jan 29 09:00–18:00 UTC Jan 30 08:00–17:00 UTC Jan 31 08:00–19:00 UTC
February
Feb 1 09:00–18:00 UTC Feb 2 06:00–15:00 UTC Feb 4 09:00–18:00 UTC Feb 6 08:00–17:00 UTC Feb 7 07:00–16:00 UTC Feb 8 07:00–16:00 UTC Feb 9 06:00–15:00 UTC Feb 11 07:00–17:00 UTC Feb 13 07:00–16:00 UTC Feb 14 09:00–18:00 UTC Feb 15 08:00–17:00 UTC Feb 16 10:00–19:00 UTC Feb 20 05:00–13:00 UTC Feb 21 07:00–17:00 UTC Feb 24 06:00–15:00 UTC Feb 25 07:00–16:00 UTC Feb 26 09:00–17:00 UTC Feb 27 07:00–16:00 UTC Feb 28 07:00–16:00 UTC
March
Mar 1 08:00–17:00 UTC Mar 2 08:00–15:00 UTC Mar 4 08:00–17:00 UTC Mar 5 06:00–15:00 UTC Mar 6 06:00–16:00 UTC Mar 7 04:00–13:00 UTC Mar 8 05:00–14:00 UTC Mar 9 06:00–16:00 UTC Mar 11 06:00–15:00 UTC Mar 13 05:00–15:00 UTC Mar 14 06:00–16:00 UTC Mar 15 07:00–16:00 UTC Mar 16 07:00–16:00 UTC Mar 18 07:00–16:00 UTC Mar 20 07:00–16:00 UTC Mar 21 04:00–13:00 UTC Mar 22 07:00–16:00 UTC Mar 23 08:00–17:00 UTC Mar 27 05:00–14:00 UTC Mar 28 05:00–15:00 UTC Mar 29 05:00–14:00 UTC Mar 30 06:00–15:00 UTC
April
Apr 1 05:00–14:00 UTC Apr 3 07:00–16:00 UTC Apr 4 05:00–14:00 UTC Apr 5 05:00–14:00 UTC Apr 6 05:00–14:00 UTC Apr 8 05:00–14:00 UTC Apr 10 05:00–14:00 UTC Apr 11 05:00–14:00 UTC Apr 12 05:00–14:00 UTC Apr 13 05:00–14:00 UTC Apr 15 05:00–14:00 UTC Apr 17 05:00–14:00 UTC Apr 18 05:00–14:00 UTC Apr 19 05:00–14:00 UTC Apr 20 05:00–14:00 UTC Apr 22 05:00–14:00 UTC Apr 24 05:00–14:00 UTC Apr 25 05:00–14:00 UTC Apr 26 05:00–14:00 UTC Apr 27 05:00–14:00 UTC
May
May 3 16:00–24:00 UTC May 4 00:00–02:00 UTC May 5 17:00–24:00 UTC May 6 00:00–02:00, 17:00–24:00 UTC May 7 00:00–03:00 UTC May 9 18:00–23:00 UTC May 10 00:00–05:00, 17:00–24:00 UTC May 11 00:00–03:00 UTC May 12 17:00–24:00 UTC May 13 00:00–03:00, 05:00–14:00, 18:00–24:00 UTC May 14 00:00–03:00, 06:00–14:00, 16:00–24:00 UTC May 15 00:00–03:00, 06:00–14:00 UTC May 16 18:00–24:00 UTC May 17 00:00–03:00, 07:00–17:00, 19:00–24:00 UTC May 18 00:00–04:00, 07:00–17:00 UTC May 19 07:00–16:00, 19:00–24:00 UTC May 20 00:00–04:00, 06:00–15:00, 18:00–22:00 UTC May 21 01:00–04:00 UTC May 22 18:00–24:00 UTC May 23 00:00–04:00, 19:00–24:00 UTC May 24 00:00–05:00, 18:00–21:00, 23:00–24:00 UTC May 25 00:00–14:00 UTC May 26 18:00–24:00 UTC May 27 00:00–14:00, 18:00–24:00 UTC May 28 01:00–04:00, 19:00–23:00 UTC May 29 01:00–17:00 UTC May 30 05:00–17:00 UTC May 31 06:00–16:00 UTC
June
Jun 1 05:00–15:00, 17:00–24:00 UTC Jun 2 00:00–03:00, 18:00–24:00 UTC Jun 3 00:00–18:00 UTC Jun 4 18:00–24:00 UTC Jun 5 01:00–14:00, 18:00–24:00 UTC Jun 6 00:00–15:00, 18:00–24:00 UTC Jun 7 00:00–14:00, 20:00–24:00 UTC Jun 8 01:00–16:00 UTC Jun 9 17:00–24:00 UTC Jun 10 00:00–03:00, 05:00–15:00, 18:00–21:00, 23:00–24:00 UTC Jun 11 00:00–04:00, 18:00–24:00 UTC Jun 12 01:00–16:00 UTC Jun 13 06:00–15:00 UTC Jun 15 01:00–03:00, 05:00–16:00 UTC Jun 16 17:00–24:00 UTC Jun 17 00:00–03:00, 05:00–14:00, 19:00–24:00 UTC Jun 18 00:00–04:00 UTC Jun 19 05:00–15:00 UTC Jun 20 05:00–15:00 UTC Jun 21 06:00–15:00 UTC Jun 22 06:00–15:00 UTC Jun 24 05:00–16:00 UTC Jun 26 04:00–13:00 UTC Jun 27 06:00–15:00 UTC Jun 28 05:00–14:00 UTC Jun 29 06:00–15:00 UTC
July
Jul 1 06:00–16:00 UTC Jul 3 05:00–14:00 UTC Jul 4 06:00–23:00 UTC Jul 5 00:00–22:00 UTC Jul 6 17:00–24:00 UTC Jul 7 00:00–03:00, 07:00–15:00, 18:00–24:00 UTC Jul 8 00:00–24:00 UTC Jul 9 00:00–13:00 UTC Jul 10 18:00–24:00 UTC Jul 11 00:00–03:00, 07:00–16:00, 18:00–24:00 UTC Jul 12 00:00–16:00, 19:00–24:00 UTC Jul 13 00:00–14:00, 17:00–24:00 UTC Jul 14 00:00–03:00 UTC Jul 15 06:00–15:00, 19:00–24:00 UTC Jul 16 00:00–16:00 UTC Jul 17 18:00–23:00 UTC Jul 18 00:00–14:00, 20:00–23:00 UTC Jul 19 03:00–14:00, 18:00–24:00 UTC Jul 20 00:00–17:00, 19:00–24:00 UTC Jul 21 00:00–04:00, 06:00–20:00 UTC Jul 22 04:00–13:00, 18:00–24:00 UTC Jul 23 00:00–05:00 UTC Jul 24 17:00–21:00, 23:00–24:00 UTC Jul 25 00:00–15:00, 18:00–24:00 UTC Jul 26 00:00–14:00, 18:00–24:00 UTC Jul 27 00:00–04:00, 05:00–14:00, 19:00–21:00 UTC