Preparing toddler for babysitter

Any advice on how to prepare my 3 year old for a whole day with a new babysitter? He’s an anxious kiddo and I don’t want to just spring it on him. So far his sitters have been family (minus when he was a wee baby so he doesn’t remember) but in a few weeks we’re helping our friends move long-ish distance and all my family is busy. He gets stressed when family watches him so this makes me nervous for him. I was thinking about asking the sitter to come over for only an hour a couple times before the big day, chat with us to show him we like them and leave briefly so he can learn the sitter is nice and fun.