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Ask her out and choose Saturday or Sunday afternoon as the time choice.  The mall should be one of the choices you can do.  That's how I always get it.  It's similar to Morgan for the water park summer date to progress her story.

Then make sure you had sex with both girls within 1 week. That should do it.  Also for full content I recommend going to 55 for both since you lose 5 yuri points doing this and one of the optional scenes require them both to be 50.

You need to date her on Saturday or sunday AFTERNOON.  There are a few date spots that are only available then and the mall is one of those.

where is your yuri level at? I know the first time I failed I didn't realize I needed this alongside the 75 lp and 25 kp.  If it is under 45 this will cause a failure as well.