Most often in April, 26 days.
Highest follower count seen that year: 35,076.
January
Jan 1 18:00–19:00 UTC Jan 2 15:00–16:00, 20:00–21:00, 23:00–24:00 UTC Jan 3 09:00–10:00, 16:00–18:00 UTC Jan 4 00:00–01:00, 16:00–17:00 UTC Jan 5 04:00–05:00, 07:00–08:00, 16:00–17:00 UTC Jan 6 02:00–07:00 UTC Jan 7 04:00–05:00, 16:00–17:00 UTC Jan 8 03:00–04:00, 06:00–07:00, 16:00–18:00 UTC Jan 9 07:00–08:00, 15:00–16:00 UTC Jan 10 18:00–19:00, 23:00–24:00 UTC Jan 11 02:00–03:00, 05:00–06:00, 17:00–18:00 UTC Jan 12 06:00–08:00, 11:00–12:00, 15:00–16:00, 23:00–24:00 UTC Jan 13 15:00–16:00 UTC Jan 14 12:00–13:00 UTC Jan 15 05:00–06:00, 08:00–09:00, 15:00–17:00 UTC Jan 21 16:00–17:00 UTC Jan 22 00:00–01:00, 05:00–07:00, 15:00–16:00 UTC Jan 23 05:00–06:00, 15:00–16:00, 23:00–24:00 UTC Jan 24 12:00–13:00, 16:00–17:00 UTC Jan 25 01:00–02:00, 15:00–16:00 UTC Jan 26 10:00–11:00, 15:00–16:00, 20:00–21:00 UTC Jan 27 00:00–01:00, 15:00–17:00, 19:00–20:00 UTC Jan 28 00:00–01:00, 04:00–05:00 UTC Jan 31 21:00–22:00 UTC
February
Feb 1 19:00–20:00 UTC Feb 2 06:00–07:00, 15:00–17:00, 21:00–22:00 UTC Feb 4 23:00–24:00 UTC Feb 7 19:00–20:00, 22:00–23:00 UTC Feb 23 05:00–06:00, 17:00–18:00 UTC Feb 25 15:00–16:00 UTC Feb 27 01:00–02:00, 09:00–10:00 UTC Feb 28 03:00–04:00 UTC
March
Mar 1 16:00–17:00 UTC Mar 2 05:00–06:00, 09:00–10:00 UTC Mar 3 14:00–16:00, 22:00–23:00 UTC Mar 4 05:00–06:00, 14:00–15:00 UTC Mar 7 19:00–22:00 UTC Mar 8 00:00–01:00 UTC Mar 9 12:00–13:00, 23:00–24:00 UTC Mar 10 19:00–20:00 UTC Mar 11 09:00–12:00, 15:00–16:00, 20:00–23:00 UTC Mar 13 12:00–15:00, 20:00–21:00 UTC Mar 16 05:00–06:00, 21:00–24:00 UTC Mar 17 12:00–13:00, 15:00–16:00, 20:00–23:00 UTC Mar 18 01:00–02:00, 05:00–06:00, 12:00–13:00 UTC Mar 19 05:00–06:00, 14:00–16:00, 22:00–23:00 UTC Mar 20 16:00–17:00, 21:00–23:00 UTC Mar 22 17:00–18:00 UTC Mar 23 05:00–06:00, 15:00–16:00, 21:00–23:00 UTC Mar 24 01:00–02:00, 05:00–06:00, 11:00–12:00, 16:00–17:00 UTC Mar 25 20:00–22:00 UTC Mar 26 05:00–06:00, 14:00–15:00 UTC Mar 28 19:00–20:00, 23:00–24:00 UTC Mar 29 17:00–18:00 UTC Mar 30 05:00–06:00, 10:00–11:00, 16:00–17:00, 19:00–20:00, 23:00–24:00 UTC Mar 31 17:00–18:00 UTC
April
Apr 1 00:00–03:00, 14:00–15:00 UTC Apr 2 04:00–05:00, 13:00–14:00, 22:00–23:00 UTC Apr 4 07:00–08:00, 15:00–16:00 UTC Apr 5 09:00–15:00, 19:00–22:00 UTC Apr 6 22:00–23:00 UTC Apr 9 13:00–14:00 UTC Apr 10 15:00–16:00, 20:00–21:00 UTC Apr 11 07:00–08:00, 11:00–12:00, 15:00–23:00 UTC Apr 12 15:00–16:00, 23:00–24:00 UTC Apr 13 02:00–05:00, 21:00–22:00 UTC Apr 14 11:00–14:00, 17:00–18:00, 20:00–23:00 UTC Apr 15 14:00–15:00 UTC Apr 16 12:00–13:00 UTC Apr 17 14:00–15:00 UTC Apr 18 06:00–07:00, 14:00–15:00 UTC Apr 19 13:00–19:00 UTC Apr 20 15:00–22:00 UTC Apr 21 15:00–16:00, 20:00–21:00 UTC Apr 22 00:00–04:00, 14:00–17:00 UTC Apr 23 16:00–17:00, 22:00–23:00 UTC Apr 24 15:00–16:00, 20:00–21:00 UTC Apr 25 05:00–06:00, 10:00–11:00, 15:00–16:00 UTC Apr 26 03:00–04:00, 12:00–13:00, 15:00–23:00 UTC Apr 27 04:00–05:00, 11:00–12:00, 15:00–16:00, 22:00–23:00 UTC Apr 28 01:00–02:00, 14:00–16:00, 18:00–19:00, 21:00–24:00 UTC Apr 30 03:00–06:00, 14:00–15:00, 23:00–24:00 UTC
May
May 1 00:00–03:00, 14:00–15:00, 23:00–24:00 UTC May 2 02:00–03:00 UTC May 8 10:00–14:00 UTC May 9 15:00–16:00 UTC May 10 13:00–19:00 UTC May 11 09:00–10:00, 14:00–17:00 UTC May 12 13:00–14:00, 17:00–18:00, 20:00–21:00, 23:00–24:00 UTC May 13 13:00–14:00, 21:00–22:00 UTC May 14 12:00–15:00, 21:00–23:00 UTC May 15 14:00–17:00, 21:00–22:00 UTC May 16 16:00–18:00 UTC May 17 00:00–01:00 UTC May 18 06:00–07:00, 14:00–15:00 UTC May 19 15:00–16:00, 19:00–20:00 UTC May 20 00:00–01:00, 03:00–04:00, 13:00–14:00, 21:00–22:00 UTC May 21 16:00–17:00, 19:00–20:00, 22:00–23:00 UTC May 22 00:00–01:00, 14:00–15:00, 21:00–22:00 UTC May 23 08:00–11:00, 14:00–18:00 UTC May 24 00:00–01:00, 06:00–07:00, 13:00–18:00 UTC May 25 13:00–14:00, 16:00–20:00 UTC May 26 00:00–05:00, 13:00–14:00, 17:00–18:00 UTC May 27 00:00–03:00, 14:00–15:00 UTC May 28 00:00–01:00, 09:00–10:00, 15:00–16:00 UTC May 29 06:00–07:00, 14:00–15:00, 21:00–22:00 UTC May 30 08:00–09:00, 14:00–15:00, 17:00–20:00 UTC May 31 13:00–14:00, 16:00–19:00 UTC
June
Jun 1 13:00–16:00 UTC Jun 2 13:00–16:00, 20:00–21:00 UTC Jun 3 14:00–16:00 UTC Jun 4 06:00–07:00, 14:00–16:00, 23:00–24:00 UTC Jun 5 15:00–16:00 UTC Jun 6 09:00–10:00, 15:00–16:00, 18:00–19:00, 23:00–24:00 UTC Jun 7 15:00–17:00, 19:00–20:00, 22:00–23:00 UTC Jun 8 13:00–16:00 UTC Jun 9 00:00–01:00, 14:00–15:00 UTC Jun 10 06:00–07:00, 15:00–16:00, 23:00–24:00 UTC Jun 11 04:00–07:00, 14:00–16:00 UTC Jun 17 09:00–10:00, 14:00–15:00 UTC Jun 18 14:00–15:00, 21:00–23:00 UTC Jun 19 01:00–02:00 UTC Jun 20 14:00–17:00, 20:00–23:00 UTC Jun 21 00:00–03:00, 14:00–15:00 UTC Jun 22 14:00–20:00 UTC Jun 23 00:00–01:00, 15:00–16:00 UTC Jun 24 14:00–15:00, 23:00–24:00 UTC Jun 25 03:00–04:00, 14:00–17:00 UTC Jun 26 01:00–02:00, 06:00–07:00, 10:00–19:00, 23:00–24:00 UTC Jun 27 15:00–18:00 UTC Jun 28 15:00–17:00, 21:00–22:00 UTC Jun 29 08:00–09:00, 14:00–16:00, 21:00–22:00 UTC Jun 30 03:00–04:00, 13:00–17:00, 19:00–24:00 UTC
July
Jul 1 00:00–01:00, 09:00–10:00, 14:00–16:00, 23:00–24:00 UTC Jul 2 02:00–05:00, 13:00–16:00 UTC Jul 3 01:00–03:00 UTC Jul 10 07:00–08:00, 12:00–13:00 UTC Jul 11 03:00–04:00, 11:00–15:00 UTC Jul 12 07:00–08:00, 14:00–20:00 UTC Jul 13 07:00–08:00, 13:00–17:00, 19:00–21:00, 23:00–24:00 UTC Jul 14 10:00–11:00, 15:00–16:00, 18:00–22:00 UTC Jul 15 21:00–24:00 UTC Jul 16 00:00–01:00, 09:00–10:00, 13:00–16:00 UTC Jul 17 06:00–07:00, 13:00–14:00 UTC Jul 18 06:00–07:00, 13:00–16:00, 23:00–24:00 UTC Jul 19 00:00–01:00, 06:00–09:00, 12:00–13:00, 19:00–20:00 UTC Jul 20 02:00–07:00, 15:00–16:00, 18:00–19:00 UTC Jul 21 00:00–01:00, 04:00–06:00, 14:00–15:00, 21:00–22:00 UTC Jul 22 10:00–11:00, 14:00–15:00, 19:00–22:00 UTC Jul 23 00:00–01:00, 04:00–05:00, 14:00–15:00, 19:00–20:00, 23:00–24:00 UTC Jul 24 01:00–02:00, 11:00–18:00, 20:00–22:00 UTC Jul 25 13:00–16:00, 18:00–19:00, 21:00–23:00 UTC Jul 26 16:00–17:00, 19:00–20:00, 21:00–23:00 UTC Jul 27 00:00–01:00, 17:00–18:00 UTC