In Project Task Gantt you can designate non-working time for assignments
By default, Project Task Gantt uses any associated work calendar to determine non-working time, such as holidays and weekends. If a work calendar is not associated with the project, Project Task Gantt indicates all weekends as non-working time.
To ignore weekends as non-working time when specifying a task duration in Project Task Gantt, check Weekends are Workdays on the Work Calendar page.