Your post is somewhat short on info, but as a general approach, I would have a single Holidays table, not two, with a check box called "Booked." When the holiday is booked, you check the box and fill in the CustomerID, or you could have a combobox based on the Customers table, and when the "Booked" checkbox is checked, have the combobox dropdown and enter the customer data that way. To force the entry of a CustomerID if the box is checked, you could make selecting an ID mandatory.
Instead of having two tables, you could have 2 forms based on queries from the Holidays table, one for booked holidays and one for available holidays. The forms and queries would be essentially identical, the only difference being whether the "booked" checkbox was checked or not.
Get the idea?
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