If you’re using custom post types, i’d save those date entries as post_meta entries (http://codex.wordpress.org/Function_Reference/add_post_meta). So if your client clicks on a date, you start an ajax request to your plugin/theme file to add or remove a meta entry.
you can create one meta_entry named _availability for example and save all dates there (you can save multiple meta_entries to a post with the same name <= 1:n)
saving your dates as meta entries has the additional benefit that you can start a WP_Query that shows you all properties available today very easily.
br from salzburg,
- Johannes