I don't know the answer to the question, but I agree with the other commenters who say it's better to tell them now than later.

I'm missing enough of the context that I don't know what kind of timescale we're talking about or how well you know the other people involved. Would they understand "I've been feeling intermittently crappy and think it might be better if I don't do this - I'd rather let you down now than at the last minute"? Some people "get" stress and mental health problems, and others don't.