TL;DR: duplicate your Date dimension, set it to "Day of Week" and then add it to your Blended Data Source.
Go to Resources > Manage added data sources > Pick your App Store data source
- Duplicate your Date dimension and call it "Date - Day of Week" and it's type to "Day of Week"
- Add the new Date dimension to your Blended Data- Add the dimension to your table
- Voilà
