"I'm sorry you're disappointed, but when I hadn't heard anything X days before the date, I assumed that you'd decided not to have a party and I made other plans."
Possibly next time when you receive a tentative invite, treat your acceptance in the same light: "Well, right NOW I'm free, and if you have a party, I'd love to come. So when you firm things up, let me know for sure, and if I'm still free at that point, put me down as a yes." The implication is that you're not reserving the night for her party until you hear for sure. Or if you think the person would still act dense or be deliberately clueless, you could say something blunt like, "I'm going to feel free to make other plans until I hear for sure that you're definitely having a party."