Date Availability Checker for Wedding Photography Website

I am a (fairly) technically competent Wedding Photographer. I am in the process of building up a new website. I need a simple form in which my clients can select a date to 'check' its availability. If the date is available I wish for it to return a email contact form structured so that is displays: Name Date Message Phone Number (Tick Box) Request Call Back Send. For dates that are unavailable I wish it return a simple apology. I am happy for the dates to be stored as ether an xml file, or a mysql file. I do not wish my clients to be able to see the dates I am busy, only check against them. The script will be embedded into my website as an iframe. So I would like it to be self contained.