The one before a nice evening with ymir. Just. After ymir saw you basically having sex with an AWAKE historia she ask if you could help her with it too. Then the suggestion of a blind fold comes up. She give you the blind fold for that night and when you get back to this do these things
Wake historia up at night
Have fun with historia.
Do some regular things with your hands then click on the highlighted circle at her eyes to let ymir have fun. then after that you can choose to have some fun with ymir. I dont remember if there was choices but if their was i chose the blowjob. To which i did that. Dont force her to deepthroat or itll ruin your relationship. be nice