What's a kind way to bow out of an event that you're asked to volunteer for, without being offensive to the person requesting?

My daughter’s school has an annual fair that’s coming up tomorrow. The parents are requested to help by working a booth. It can be fun but it’s also an obligation.

I am friendly with the moms but not friends with them and I don’t see them too often, as I’m busy working most of the time.

I am unable to volunteer for the fair because I have a sick relative that I will be spending the day with. I don’t feel like explaining those details, but don’t want to be curt and just say “I’m unable to attend.”

Is there a nice way to say that I am unable to volunteer without being abrupt?

An option I am thinking about is just saying “We weren’t planning to attend the fair.”

What do you think?