To be honest I have had many similar experiences like yours whilst being someone who has social anxiety. Not all of the experiences were the same, but I would some days be 'full of life' and have the motivation to go out of my boundaries to do stuff I usually avoid and on impulse I would make decisions like agreeing to meet up, sign up to a competitions or courses, arrange trips with friends etc. Later on when all the energy is dried up I would be filled with regret. Sometimes I would try to cancel whatever I planned using some excuse, but other times I would still go forward with it and when it is all over I usually found myself to have enjoyed the experience.
In short, it's your mindset and how you view it that makes you feel how you currently are. In addition, it may just be a temporary mental resistance from you doing something out of your comfort zone and when the ball starts rolling those feelings may reduce. Try sorting out your thoughts on how and what you will be doing over the 2 days and maybe your dread (if caused by uncertainty) may be reduced.
Hope it helps.