This was driving me crazy too....I even phoned Airbnb and they weren't able to help!!!!
I simply wanted to set a weekly price.
(I will check this when I set up another listing ....but here for starters is how I believe I did it)
1. Go into your calendar and select "pricing settings" on the right
2. Scroll down and just under the monthly discount options click Custom weekly price.
3. You can now select dates and input prices
Having done this you may want to say the customers can only stay 7 nights and arrive on a specific day
1. Click "Availability settings" on the right
2. Scroll down and select "Add another requirement"
3. Click the drop down arrow in the "During" box and select "specific dates"