If mom is a a&& and decides to pick daughter up that weekend, make sure you are there early so she will see you and get that you are picking up daughter. Of course you do not want a scene, just stand your ground, and escort daughter to your car. If mom does cause a scene, and tries to obstruct you or daughter, I would call police then and request a escort. I had to do that sometimes, and the police would do just that, escort myself and child to our car without interference.
If it were me, not saying its the best judgement call, I would send a fresh email to mom, state as a reminder or something, you are taking daughter to....on YOUR weekend with daughter, address, contact number.
State that it is disappointing child is missing a extracurricular activity, however significant family event and that your time with daughter it is you who makes the decision as to what is in her best interests.
Thank her for her cooperation around this matter, sign off. And THEN don't write back. This way, since you sounded unclear if ex was clear whether you would take child for the weekend, she gets that you ARE clear.
My two cents, worth no more.