Most often in March, 26 days.
Highest follower count seen that year: 12,965.
January
Jan 1 12:00–16:00 UTC Jan 3 12:00–18:00 UTC Jan 4 11:00–18:00 UTC Jan 7 12:00–19:00 UTC Jan 8 11:00–17:00 UTC Jan 9 12:00–18:00 UTC Jan 10 11:00–19:00 UTC Jan 11 10:00–18:00 UTC Jan 13 13:00–18:00 UTC Jan 14 12:00–18:00 UTC Jan 15 13:00–19:00 UTC Jan 16 15:00–19:00 UTC Jan 17 12:00–18:00 UTC Jan 18 11:00–16:00 UTC Jan 20 14:00–19:00 UTC Jan 21 13:00–19:00 UTC Jan 22 13:00–18:00 UTC Jan 23 11:00–17:00 UTC Jan 24 11:00–18:00 UTC Jan 25 11:00–17:00 UTC Jan 27 12:00–19:00 UTC Jan 29 13:00–18:00 UTC Jan 30 12:00–18:00 UTC Jan 31 12:00–18:00 UTC
February
Feb 1 12:00–18:00 UTC Feb 4 14:00–19:00 UTC Feb 5 11:00–15:00 UTC Feb 6 11:00–17:00 UTC Feb 7 11:00–17:00 UTC Feb 8 10:00–18:00 UTC Feb 10 10:00–13:00, 15:00–16:00 UTC Feb 11 07:00–09:00, 16:00–19:00 UTC Feb 12 13:00–18:00 UTC Feb 13 11:00–19:00 UTC Feb 14 12:00–19:00 UTC Feb 15 12:00–17:00 UTC Feb 17 12:00–19:00 UTC Feb 18 12:00–19:00 UTC Feb 19 12:00–19:00 UTC Feb 20 12:00–19:00 UTC Feb 21 12:00–19:00 UTC Feb 22 12:00–18:00 UTC Feb 24 11:00–19:00 UTC Feb 25 11:00–19:00 UTC Feb 26 12:00–17:00 UTC Feb 27 12:00–19:00 UTC Feb 28 12:00–18:00 UTC
March
Mar 1 10:00–14:00 UTC Mar 3 10:00–18:00 UTC Mar 4 12:00–19:00 UTC Mar 5 12:00–18:00 UTC Mar 6 10:00–18:00 UTC Mar 7 13:00–18:00 UTC Mar 8 11:00–17:00 UTC Mar 10 15:00–18:00 UTC Mar 11 10:00–12:00, 14:00–18:00 UTC Mar 12 14:00–19:00 UTC Mar 13 13:00–19:00 UTC Mar 14 11:00–18:00 UTC Mar 15 10:00–17:00 UTC Mar 17 12:00–19:00 UTC Mar 18 13:00–19:00 UTC Mar 19 11:00–18:00 UTC Mar 20 11:00–19:00 UTC Mar 21 12:00–19:00 UTC Mar 22 11:00–17:00 UTC Mar 24 10:00–17:00 UTC Mar 25 12:00–19:00 UTC Mar 26 13:00–18:00 UTC Mar 27 11:00–18:00 UTC Mar 28 12:00–19:00 UTC Mar 29 11:00–18:00 UTC Mar 31 10:00–12:00, 14:00–17:00 UTC
April
Apr 1 08:00–11:00, 13:00–18:00 UTC Apr 2 10:00–17:00 UTC Apr 3 09:00–17:00 UTC Apr 4 10:00–16:00 UTC Apr 5 10:00–16:00 UTC Apr 8 11:00–18:00 UTC Apr 9 10:00–17:00 UTC Apr 10 11:00–18:00 UTC Apr 11 10:00–17:00 UTC Apr 12 07:00–15:00 UTC Apr 14 11:00–16:00 UTC Apr 15 11:00–18:00 UTC Apr 16 13:00–17:00 UTC Apr 17 12:00–17:00 UTC Apr 18 13:00–19:00 UTC Apr 19 06:00–08:00, 10:00–16:00 UTC Apr 21 11:00–13:00, 15:00–17:00 UTC Apr 22 11:00–18:00 UTC Apr 23 11:00–19:00 UTC Apr 24 12:00–19:00 UTC Apr 25 12:00–18:00 UTC Apr 26 10:00–16:00 UTC Apr 29 08:00–18:00 UTC Apr 30 10:00–15:00, 18:00–19:00 UTC
May
May 1 09:00–17:00 UTC May 2 11:00–12:00, 15:00–16:00 UTC May 3 11:00–16:00 UTC May 5 11:00–18:00 UTC May 6 10:00–13:00 UTC May 7 15:00–19:00 UTC May 8 12:00–17:00 UTC May 9 12:00–18:00 UTC May 10 09:00–15:00 UTC May 12 13:00–17:00 UTC May 13 11:00–18:00 UTC May 14 12:00–17:00 UTC May 16 11:00–18:00 UTC May 17 09:00–14:00 UTC May 19 17:00–18:00 UTC May 20 08:00–10:00, 12:00–18:00 UTC May 21 11:00–13:00, 16:00–18:00 UTC May 23 11:00–18:00 UTC May 24 11:00–17:00 UTC May 26 13:00–19:00 UTC May 27 13:00–18:00 UTC May 29 11:00–17:00 UTC May 30 12:00–18:00 UTC May 31 10:00–17:00 UTC
June
Jun 2 13:00–19:00 UTC Jun 3 12:00–17:00 UTC Jun 4 14:00–19:00 UTC Jun 6 12:00–18:00 UTC Jun 7 11:00–14:00, 16:00–17:00 UTC Jun 11 16:00–17:00 UTC Jun 12 10:00–19:00 UTC Jun 13 14:00–18:00 UTC Jun 16 09:00–14:00, 16:00–18:00 UTC Jun 17 11:00–18:00 UTC Jun 18 12:00–16:00 UTC Jun 20 10:00–18:00 UTC Jun 21 10:00–17:00 UTC Jun 24 10:00–18:00 UTC Jun 25 15:00–19:00 UTC Jun 26 09:00–16:00 UTC Jun 27 11:00–19:00 UTC Jun 28 08:00–11:00, 13:00–16:00 UTC
July
Jul 2 09:00–19:00 UTC Jul 3 11:00–18:00 UTC Jul 4 11:00–17:00 UTC Jul 5 09:00–17:00 UTC Jul 8 10:00–17:00 UTC Jul 9 10:00–17:00 UTC Jul 10 13:00–18:00 UTC Jul 11 10:00–17:00 UTC Jul 12 10:00–17:00 UTC Jul 14 11:00–18:00 UTC Jul 15 10:00–15:00 UTC Jul 19 09:00–16:00 UTC Jul 20 11:00–20:00 UTC Jul 22 12:00–18:00 UTC Jul 23 14:00–19:00 UTC Jul 24 10:00–15:00 UTC Jul 25 11:00–17:00 UTC Jul 26 11:00–14:00, 15:00–16:00, 17:00–18:00 UTC