"Hi Jaggerdagger. Just wondering if you're doing anything next Saturday?"
But then... THEY DON'T SPECIFY what they want you to be free for! Could be anything. I might be free on Saturday, but I certainly don't want to go to a Rod Stewart tribute act, for instance. Or mud flinging in Gloucestershire when I live in Essex. Or even going to a snotty nosed child's birthday party on my only day off, when I'm a primary school teacher.
Is it just me that finds this really irritating, and that one should always state the full facts in the initial message?