Datepicker beforeShowDay

Hi,
Working on a booking form, and I want a function to check if dates
are available for booking.
Seen some examples, but can´t get it right.
I´m looking for a function that check a range of dates(eg, 2009-10-12
to 2009-11-10) and then set them available or not.
Any suggestions on a function that can do that?