I saw some comments why we assume that
1st big hetero sex scene with an important character: Within the next 4 playable days
2nd big hetero sex scene with an important character: Within the next 4 playable days
will means exactly PD 16 and not any day before that.
1.PD 15-we can forget about it-it's Sophia/Patricia day
2.The dev would use "Within the next 1/2 playble days" for PD 13/14
3.We know that there won't be any sex scene with Sophia in the first half of the game.PD 16 is first day of the second half.
4.No way hetero sex scene to be before at least one lesbian sex scene(PD 15).