From the Pricing / Price plans screen, you'll see a column showing the Minimum Length of Stay (known as MinLOS) that a guest needs to stay in order to qualify for a particular price.
In the screenshot below, we have simple pricing for one night, where every night is priced the same, irrespective of how long a guest stays.
Discounting longer stays
Using the blue "+" button to the left of the 1, we can add additional MinLOS pricing:
Now, if a guest books a double room for any 2 nights, they would qualify for the reduced nightly rate of £55/£65 for single/double occupancy.
Enforcing a 2 night stay on Saturday.
By removing the 1 night MinLOS pricing for Saturday, and having only a 2 night price available, the guest is unable to book Saturday for 1 night.
They must book Friday and Saturday (at £60+£75) or Saturday and Sunday at £75+£55).