A clear “no” now is kinder than a fake “yes” you’ll back out of Thursday.

The yes you do not mean is not a favor, it is a delay that lands on somebody else. They stop looking for another option, build the plan around you, and then find out on Thursday with no time left. Saying no today hands them everything they need. It is less comfortable for you in the moment and much cheaper for them.

If you already know you are not going, say so now rather than waiting for a better moment, because that moment never turns up. One line is enough and you do not owe an explanation. People forgive an early no far faster than a late cancellation.