Okay so I figured out how to get the special Sakuya scene! It played on March 12th while I had a Star, S+, S and A in my stats. I've tried phoning Sakuya, but nothing different happened.
I'm going to try getting all 5 stars and hearts for the heroines and get this scene at the same time to see if that's how you get 7th ending, it's going to be hard though.
[Addendum] The only requirement needed for the Sakuya scene is that you have a 500 in charisma.