The calendar you import to your User Account overwrites the availability settings of your Wimdu calendar. Any date that is blocked by the imported calendar cannot be changed to 'available' using the Wimdu calendar. To make such changes, you need to make the day available in your external calendar. However, you can still block the availability of your property directly through the Wimdu calendar. These changes will not show in your external calendar.
We recommend that you keep your calendar synchronized in order to ensure it is updated at all times.