@Lala89 You may want to adjust your pricing to attract longer stays - for example, when I start a listing, my weekday pricing is lower than my weekend pricing, (180 weekday, 200 weekend) and my week discount is 7 times my weekday price so guests would "get a great deal" if they stayed longer. Note: I did not discount my weekday price lower than what I wanted -- I increased my weekend price. I still had lots of weekend only bookings, but started to see more weekdays get booked, so it was a win-win.
If you only want 7 day bookings, change your listing to 7 days with a designated turnover day. This will only work well if your area attracts week-long visitors (like most beach locations do). I have also seen duplicated property listings where one listing is open only on the weekends and the other listing for the same property is only for the weekdays.