Most often in March, 31 days.
Highest follower count seen that year: 8,387.
January
Jan 1 10:00–15:00 UTC Jan 2 10:00–14:00 UTC Jan 3 09:00–14:00 UTC Jan 4 10:00–14:00 UTC Jan 5 09:00–12:00 UTC Jan 6 11:00–14:00 UTC Jan 7 10:00–14:00 UTC Jan 8 10:00–14:00 UTC Jan 9 10:00–14:00 UTC Jan 10 09:00–14:00 UTC Jan 11 07:00–14:00 UTC Jan 12 07:00–13:00 UTC Jan 13 07:00–13:00 UTC Jan 14 08:00–11:00 UTC Jan 15 07:00–08:00, 10:00–11:00 UTC Jan 16 08:00–13:00 UTC Jan 17 07:00–09:00 UTC Jan 18 07:00–11:00 UTC Jan 19 08:00–10:00, 16:00–17:00 UTC Jan 20 08:00–13:00 UTC Jan 21 09:00–15:00 UTC Jan 22 08:00–13:00 UTC Jan 23 08:00–11:00 UTC Jan 24 07:00–13:00 UTC Jan 25 08:00–10:00, 16:00–18:00 UTC Jan 26 08:00–11:00, 15:00–16:00 UTC Jan 27 08:00–13:00 UTC Jan 28 08:00–11:00, 13:00–15:00 UTC Jan 29 08:00–10:00, 15:00–17:00 UTC Jan 30 08:00–09:00 UTC
February
Feb 1 08:00–12:00 UTC Feb 2 08:00–10:00, 15:00–16:00 UTC Feb 3 08:00–12:00 UTC Feb 4 13:00–15:00 UTC Feb 5 16:00–17:00 UTC Feb 6 10:00–13:00 UTC Feb 7 08:00–09:00, 14:00–16:00 UTC Feb 8 08:00–09:00 UTC Feb 9 07:00–10:00, 16:00–17:00 UTC Feb 10 07:00–13:00 UTC Feb 11 08:00–13:00 UTC Feb 12 07:00–14:00 UTC Feb 13 08:00–13:00 UTC Feb 14 10:00–13:00 UTC Feb 15 09:00–12:00 UTC Feb 16 08:00–13:00 UTC Feb 17 07:00–10:00 UTC Feb 23 07:00–13:00 UTC Feb 24 07:00–13:00 UTC Feb 25 07:00–09:00, 11:00–14:00 UTC Feb 26 08:00–14:00 UTC Feb 27 08:00–14:00 UTC Feb 28 08:00–13:00 UTC
March
Mar 1 09:00–15:00 UTC Mar 2 07:00–12:00 UTC Mar 3 07:00–12:00 UTC Mar 4 08:00–09:00 UTC Mar 5 08:00–11:00, 14:00–15:00 UTC Mar 6 07:00–12:00 UTC Mar 7 09:00–12:00 UTC Mar 8 08:00–14:00 UTC Mar 9 07:00–13:00 UTC Mar 10 08:00–12:00 UTC Mar 11 07:00–13:00 UTC Mar 12 10:00–14:00 UTC Mar 13 08:00–13:00 UTC Mar 14 09:00–12:00 UTC Mar 15 08:00–14:00 UTC Mar 16 08:00–13:00 UTC Mar 17 08:00–14:00 UTC Mar 18 08:00–13:00 UTC Mar 19 07:00–13:00 UTC Mar 20 07:00–13:00 UTC Mar 21 09:00–14:00 UTC Mar 22 07:00–13:00 UTC Mar 23 07:00–13:00 UTC Mar 24 07:00–13:00 UTC Mar 25 07:00–13:00 UTC Mar 26 08:00–13:00 UTC Mar 27 08:00–12:00 UTC Mar 28 08:00–14:00 UTC Mar 29 07:00–14:00 UTC Mar 30 08:00–13:00 UTC Mar 31 08:00–12:00 UTC
April
Apr 1 08:00–13:00 UTC Apr 2 08:00–13:00 UTC Apr 3 07:00–12:00 UTC Apr 4 07:00–13:00 UTC Apr 5 08:00–12:00 UTC Apr 6 06:00–11:00 UTC Apr 7 06:00–11:00 UTC Apr 8 06:00–12:00 UTC Apr 9 07:00–13:00 UTC Apr 10 08:00–13:00 UTC Apr 11 09:00–13:00 UTC Apr 12 11:00–15:00 UTC Apr 13 08:00–14:00 UTC Apr 14 08:00–13:00 UTC Apr 15 08:00–13:00 UTC Apr 16 09:00–13:00 UTC Apr 17 06:00–12:00 UTC Apr 18 07:00–13:00 UTC Apr 19 08:00–11:00 UTC Apr 20 07:00–12:00 UTC Apr 21 07:00–12:00 UTC Apr 22 06:00–12:00 UTC Apr 23 07:00–13:00 UTC Apr 24 07:00–11:00 UTC Apr 25 06:00–13:00 UTC Apr 26 08:00–13:00 UTC Apr 27 08:00–12:00 UTC Apr 28 09:00–11:00 UTC Apr 29 08:00–12:00 UTC Apr 30 08:00–14:00 UTC
May
May 1 08:00–10:00 UTC May 2 09:00–12:00 UTC May 3 07:00–12:00 UTC May 4 07:00–09:00, 11:00–12:00 UTC May 5 10:00–13:00 UTC May 6 08:00–13:00 UTC May 7 09:00–13:00 UTC May 8 08:00–13:00 UTC May 9 09:00–13:00 UTC May 10 08:00–13:00 UTC May 11 08:00–13:00 UTC May 12 08:00–13:00 UTC May 13 09:00–14:00 UTC May 14 09:00–13:00 UTC May 15 08:00–13:00 UTC May 16 06:00–12:00 UTC May 17 08:00–12:00 UTC May 18 07:00–12:00 UTC May 19 08:00–13:00 UTC May 20 07:00–13:00 UTC May 21 08:00–12:00 UTC May 22 08:00–12:00 UTC May 23 06:00–11:00 UTC May 24 08:00–13:00 UTC May 25 09:00–11:00, 13:00–14:00 UTC May 26 08:00–11:00 UTC May 27 07:00–12:00 UTC May 28 09:00–13:00 UTC May 29 08:00–12:00 UTC May 30 09:00–12:00 UTC May 31 08:00–11:00 UTC
June
Jun 1 09:00–13:00 UTC Jun 2 08:00–13:00 UTC Jun 3 09:00–12:00 UTC Jun 4 08:00–13:00 UTC Jun 5 10:00–12:00 UTC Jun 6 08:00–14:00 UTC Jun 7 11:00–15:00 UTC Jun 8 08:00–13:00 UTC Jun 9 09:00–13:00 UTC Jun 10 09:00–13:00 UTC Jun 11 09:00–12:00 UTC Jun 17 07:00–12:00 UTC Jun 18 09:00–13:00 UTC Jun 19 11:00–13:00 UTC Jun 20 07:00–12:00 UTC Jun 21 08:00–11:00 UTC Jun 22 08:00–12:00 UTC Jun 23 07:00–11:00 UTC Jun 24 08:00–12:00 UTC Jun 25 07:00–10:00 UTC Jun 26 08:00–11:00 UTC Jun 27 11:00–12:00 UTC Jun 28 07:00–08:00 UTC Jun 29 09:00–11:00 UTC
July
Jul 1 07:00–13:00 UTC Jul 2 08:00–11:00 UTC Jul 3 07:00–12:00 UTC Jul 4 07:00–12:00 UTC Jul 5 09:00–12:00 UTC Jul 6 09:00–12:00 UTC Jul 7 07:00–11:00 UTC Jul 8 08:00–12:00 UTC Jul 9 08:00–13:00 UTC Jul 10 08:00–14:00 UTC Jul 11 09:00–13:00 UTC Jul 12 06:00–11:00 UTC Jul 13 09:00–13:00 UTC Jul 14 09:00–11:00 UTC Jul 15 12:00–13:00 UTC Jul 16 08:00–13:00 UTC Jul 17 08:00–12:00 UTC Jul 18 08:00–12:00 UTC Jul 19 07:00–12:00 UTC Jul 20 09:00–13:00 UTC Jul 22 12:00–14:00 UTC Jul 23 10:00–15:00 UTC Jul 24 09:00–13:00 UTC Jul 25 06:00–07:00 UTC Jul 27 09:00–13:00 UTC