Usually, you set up a calendar table of all Saturday dates in the range
you are interested in.  Then compare the booking table against the
calendar table.  Sorta like this:

CREATE TABLE calendar (saturday_date DATETIME NOT NULL);

[Just example dates - not actual Saturday dates]

INSERT INTO calendar VALUES (#1/1/2006#);
INSERT INTO calendar VALUES (#1/8/2006#);
INSERT INTO calendar VALUES (#1/15/2006#);
... etc. ...

Then the query looks like this:

SELECT c.saturday_date
FROM calendar As c LEFT JOIN booking as b
     ON c.saturday_date = b.book_date
WHERE b.book_date IS NULL

The LEFT JOIN combined w/ the WHERE clause produces all dates in the
Calendar table (Saturday dates) that are not in the Booking table.