"Wow, I bet So-andso will be REALLY excited about this <insert gratuitous gift grab, or any event you don't feel like attending here>. Unfortunately, I will not be able to attend. Send So-and-so my best."
leave it at that. Unfortunately, I will not be able to attend is my best friend, if they ask why I cannot be there- "I wouldn't want to bore you with the details, but have fun!" or "It just isn't going to work for me, sorry."
Simple, honest, and to the point. No having to keep track of what you need to do that day.
ETA-It's pro'lly fairly obvious that I read the OP wrong. If someone is offerring to host an event FOR you, and you do not want it to happen, simply tell them "Thank you for the offer, and I am glad you care, but I would prefer not to have said event." after that, they can respect your wishes, or it's out of your hands.