Thanks for playing!
On your questions (spoilers ahead!):
Sakuya's event is triggered by having 500 charisma [star rank] before March 12th nighttime (though to be honest, I'm not sure how you got it with ~400 charisma [S rank] instead).
This is the "0.1st" ending the description refers to despite not actually being an ending.
The main four heroines each have a special date they can ask you out on if you have roughly 4+ hearts (roughly 67% total affection) and 5 stars (stat requirements fully met) with them.
They can schedule this via the random post-exploration events if these conditions are met.