Hi,

probably just write out that big html file with every date linked to sth like
cal.php?action=chg&day=17&month=3&year=2012
You would probably want to setup a database that combines a date with some id - it is quite likely that your client wants to store information about the visitors in the database as well, and that id column would link to the visitors table. If a day is not booked, there simply would be no entry for that date
BTW: it seems that the initial state of every one day should be available, rather than unknown.

Musicman