SimplyBook.me has no option of booking days, however, there is a workaround, please check if it is suitable for your business.
1. In Settings // General Settings // Configuration // Allow service to extend over breaktime and Allow service to extend over closing hours.
2. In Manage // Services // "+" button create service with the duration corresponding to the amount of days your service will be offered for (3 days=3 hours, 7 days=7 hours).
3. In the Time settings // Company opening hours (when you have only 1 service) or in the working hours of this service (Time settings // Service schedule or Manage//Services//select the service//Service schedule) open only 1 hour each day.
4. The system will then book the number of hours indicated in service duration as the number of days.