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(new interface) or Manage // Services // Add service button(old interface) 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 the provider (Time settings // Service provider schedule) related to the service (see how to relate service and provider here ) 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.