Sorry, I can't make it

I was recently invited to some event or other. This was my reply.

I'm sorry, I can't make it;
The plumber's coming round.
He's got to fix a leaky pipe
That makes a dripping sound

And if he doesn't make it,
Then I'm going to a gig
I don't know who is playing
But I'm sure they're pretty big.

And if the gig is cancelled,
Then I've got to wash my hair.
It isn't very dirty
But I'd best, while it's still there!

So you see, that day I'm busy.
There's not one minute I can find!
But I do just have one question:
What's the date you had in mind?