Hi @richmont,
No worries, I can just give you a hint then 
You can do followings:
- Create calculated column [Workday Not Holiday] in Date table. Make it 1 if it is workday and 0 if it is weekend.
- Create calculated column in your orders table etc where you count no. of days between order and delivery.
- From here, you can easily pick up which order are late etc.
You can take below example how to create calculated columns for workday/public holiday etc from this .pbix file.