I can relate somewhat to the situation. Was in a flare, then after I started feeling better, I tried some light housecleaning (mostly waving a feather duster around and sweeping up any dust bunnies that were larger than the dog

), and guess what? I'm back in Flare-ville!
If you think you will feel ok for the party, then go ahead with it but don't worry about the housekeeping. Maybe if one of your guests is a really good friend he/she can come early and tidy a little for you?? Maybe only have them tidy the kitchen because you'll be needing that and shut the doors on any rooms you won't be using so the other guests won't see them?
But if you suspect you're not gonna be up for the party regardless of whether or not anything gets done to the house (the world will not end if you don't clean), the considerate thing to do would be to cancel and let everyone know ASAP. I know it's frustrating to not be certain how you'll feel on a given day, but hey, you have fibromyalgia, so you have to go with your best guess. If you do end up canceling, can you co-host a party with someone sometime so you can still have the experience but not have to do it all yourself?
I really hate what fibro does to the social life. I ended up going to a family get-together Sunday with a migraine. I didn't want to cancel because there were people visiting from out of town that I hadn't seen in four years, but I was so out of it I could barely form a sentence.
Hope something in here helped.
fanfaire