Hello, I know that you addressed Siren but since I know the answer that you seek, I take the initiative to give it to you so you have it right away and Siren can concentrate on the game's development progress, I hope you don't mind? :-)
In order to attend the flower ceremony in the gardens, you have to get the flower charm from the gardens then go to the bunny mansion to add the salacious sakura tale then select it if you meet the relationships requirements. :-)