A Free Night On decreases the cost of the booking by the Nightly Rate. The number specified in the Free Night On box dictates which Nightly Rate is removed. 

For example, if 3 is placed in the Free Night On box, the Nightly Rate for the third night will not be charged (and effectively the guest will be charged 3 nights for the price of 2).

Let's see that in action:

Did this answer your question?