You can set price and availability for each individual day via the calendar. The pricing you set in the calendar overrides your default price per night.
To get to the calendar, simply login and hit the MANAGE button for your van. Then click the CALENDAR button at the top of the page.
From here you can select a day (or multiple), and set a price for those day(s). You can also mark those days as unavailable.
To edit the default price for your van, hit MANAGE, then click Edit on the "Price and Terms" section. Here you'll see a form for default price -as well as service fee and minimum nights.