Generally the system offers the lowest rate valid for each night.
There is an exception if "Do Not Undercut" is selected for any rate but we will assume this is not the case in this example.
Set your highest (or rack) price in one rate and make it available on all days. (this is the default price if no other price applies)
Then create a second discounted rate and make it available only on the days you desire by deselecting the "Allowed Nights" checkboxes on the days it does not apply.
Optionally create more rates with different discount prices on other nights.
You can use the minimum stay, bookable starting and bookable ending settings with each rate to combine minimum stay periods, advance or last minute booking rules with your discounted prices and create sophisticated rate rules.