I have never cancelled because of anything stressful in my life. I don't get easily stressed anyway and for me it would be more stress not to go to my therapy session. For instance,

I went directly from my session to my PhD defence (in my country, the custom is to have a public defence: presentation, discussion with the opponents and then a secret voting by the committee). By no means I would have wanted to cancel before that even if I could. (But I can't anyway - I'm in intensive several times per week therapy and I can only miss sessions because my T does not accept cancellations).

Also, I don't plan ahead what we'll talk about - I let things just occur. I've found that I can't force myself to work on difficult topics by will because if I try to do that it frustrates me more than helps. At the same time, I never use the session to give overview about my everyday life either.

So, I don't know. I wouldn't cancel. I would just go in and see how it goes. If it feels like diving in then I would do that and if it feels right to keep it light I would do that.