What is prohibited is to single out Friday or Saturday by fasting.
Fasting on Friday or Saturday  is permissible if you fast day before it or
after it so that you don’t single it out.

It was said by the prophet (S.A.W) that fasting on yaohm jumu’ah and sabt is prohibited except on itsnain and khamiz, it came from another hadeeth that it is sunnah to fast on 13,14 and 15 of lunar month. Here is my question,what if this three days falls under the days we are not allowed fast, how can we treat this hadeeth?