How and when do you bring up the subject of having an invisible chronic disease?

[mopinko]

2. yup. just had to explain that to someone, and

i was thinking about making a larger announcement to friends, etc. that if i say i might be there, i might not. i just can't predict what will happen after 5 pm, and try not to have to.
i do usually say i was tired, but sometimes include whatever ridiculous circumstance occurred today that ruined my evening.