If your customer is closed during the weekend or if there is nobody from your warehouse/transportation planning department on Saturday/Sunday, then what is the point in having a delivery created automatically on that day?
By the way, if there is a delivery for Saturday and the plant is closed, the document: 1) is considered overdue, which impacts people's KPI and salaries; 2) there is a risk that the delivery will not be included for further processing because of default selections.
As to the specific issue, I would start with checking factory calendars in the org. structure, goods receiving hours of customers (and the calendar of course), routes (there is also a calendar), working times, shifts... and by checking I do not mean only in the system, but also confirming with the business the correct\actual values.
If you use CRM as well, make sure to keep your settings consistent.