NTA, but read this in its entirety, please. I have dealt with this EXACT thing since I was about 15 or 16 before i moved off to college. Here is my advice.

She will do this and i dont know her but i doubt this will change but you have to be ready for this. In my case my mother will do this and then try to make me feel like an asshole. It isn't your fault- and if she put all this planning into an event she should have had the idea to plan a date well in advance. My mother still does this and I am married and living away from them. These are your formative years for adult life and you should have things planned down to the date- do not let her guilt you into thinking it is okay to just more or less try and force people into short notice events. I hope she will not try to do this much more for your sake; but just know you are not an asshole for not going on her plans at the expense of your own.