This response helps because…
There will be some people you want to share your health details with, but keeping lots of people updated will mean spending a disproportionate amount of time talking about cancer, leading to an unnecessarily heavy dose of unpleasant emotions.
People don’t want to feel shut out, so saying “today’s not a good day” makes it temporary.
People want to know how they can help, so giving them permission to distract you by talking about themselves helps satisfy them.
People often shy away from sharing their problems with someone going through a large stressor like cancer. Yet hearing other people’s problems makes us feel better. That’s because being a good friend is part of who we are, and also, we don’t want to be the only one with a problem! Give your friends permission to share their own troubles with you.