On 11/22, speak with Yosuke in the 2-2 Classroom.
On 11/23, speak with Chie in the Shopping District, North.
On 11/24, speak with Rise on the 1st Floor of the Classroom Building.
There may be other days and classmates who you can talk to trigger the hospital visit. It MUST be done before Nanako arrives back at home.
All of your Classmates will NOT have the ! Mark above their head when you can visit Nanako. But they will ask if you want to see Nanako today.
You do get 9 Points to all party member Social Links for doing this.
This does not impact the story in anyway.