Query to get dates NOT in table
To do what you want first step is generate a list of Dates  then join that reservation table to the generate list of dates.   

Here is a thread on how to Generate list of Dates in MSSQL

From this result set select where List of dates table failed to joined to the reservation table.  The reservation columns will be filled with NULLs

To write a sample  query for you, have to know the structure the table and unique keys used to identify the records.