...
This only applies to the standard (e) notification emails. It does not apply to the Preview Email notifications.
Configuration option at a project level where you can customize and define which days are the working days within a week.
This option can be located under Configure > Project details
You can to define the work start and end date by picking a week start day and week end day
The days outside of this will be considered the weekend
These weekend days are what will be considered in the due date configuration for the weekend.
For eg
Mon-Fri, means Sat-Sun are weekend
Mon-Sat, means Sun is the only weekend
Sun-Thu, means the weekend is Fri-Sat and so on