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. 

To set this up you need to go to Prices > Price update > New Price Update

1. Select the Free night on from the dropdown box (if you can't see this refer to our guide on configuration settings)

2. Put the night you want free in the value box

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).

3. Add the rooms, date range and days of the week you want this to apply to.

4. Then press Set prices

Let's see that in action:
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For other types of promotions see our guide on setting up Promos.