Depending on the day and how busy it it you might be able to sign up in the afternoon.
We went to HS May 1, anniversary day. We had a PPO breakfast so go to sign up early and got the first show but when we got back to the park after a break later in the day there were still spots available for the last show so my daughter actually did it twice that day.
ETA: For the last show they were still signing people up while those who had previously signed up were checking in.