Women are replaceable. If you have this attitude, you will bag a lot of chicks.
To the OP, she texted you last minute because her "plan A" fell through and you are the 2nd choice (or 3rd..) for the evening. Event though you want to bag her, resist the urge to agree to do something that same day. She will forever regard you as someone who is not worth her complete attention, as you have proven that you will drop everything and spend time with her that night, last minute and on her schedule.
I understand that you wish to boan her. If this is still the case, you must, as another poster suggested, tell her you are busy that night but suggest that next weekend will be OK, and that YOU
will get back to her later next week. Don't call her (NO texting, it's for punks) until the day right before you want to see her, such as Thurs night for Fri night. This way she will have spent all week wondering if you are going to call, and be thinking of you all week, thus upping your desireability in her head.
I CANT believe I even have to type this out for you people. I should teach a class or something. But alas I was also just as clueless back when I was in my early 20s.
Again, in general minimize the texting and make calls instead. Texting is for juvenile boys who have no game. It's stuff like this that makes girls easy pickings for older 'more mature' men like me. Or continue doing what you are doing, and keep men like me freshly supplied with young mid 20s women for our weekend entertainment