LOL... I'm the opposite of
@SheilaKathy. The things I dread are usually a very timely warning that I am not in a good place and things are going to go very badly if I try and push myself. I have had a few things surprise me, as not being as bad as I was worried about. But that's stacked up against hundreds of times things were far worse. I am open to things going better than I fear, but I plan for them going worse.
As such, if I'm able, I usually reschedule/cancel/something along those lines. Can't do that for a lot of things (my own wedding, for example ;) although maybe I should have!).
If canceling is not an option (one's own wedding, court appearances, etc.)... Major self care. Slavish adherence to everything that works well in my life (routines, exercise, pacing myself, grounding, taking breaks, etc.) and changing other things in my life to work around it. Like hiring a sitter & taking a day off work to spend a day sleeping/ relaxing/ building reserves... And ditto the day after (or several days) in order to recover.
If canceling is not
preferred (someone else's shindig), but it's also not a "life-stopper" event that I need to be taking time off & completely rearranging my life in order to make it happen... I work with what I've got. Meaning I may make an appearance. Or I may go, but be ready to plead migraine, in order to take breaks. Or I may plan on only going if and only if I feel up to it. Or, or, or. It's much more of a balancing act.