By default, when you schedule a task, only working days (typically weekdays) are taken into account. For example, if you schedule a task for three days and the task’s start date is Friday, then the task’s end date will be Tuesday of the following week.
Open the task in Task view. You can do that right from the Gantt chart.
Click the Set Date button or the dates for which the task is to be scheduled.
Uncheck the Working days only option (located at the bottom of the scheduling pop-up).
If you change the task start/end dates from the table or within the expanded task before unchecking Working days only, the non-working days and vacations will be deducted from the total duration of the task.
After you change task settings, you can now change the task dates and duration, thus scheduling it on weekends.
Non-working days and vacations are also factored in when you reschedule a task. If you drag the task or change the task start/end date(s), and the assignee has vacation or non-working days planned in that period, you'll see the number of days off that affect the task’s duration near the assignee’s name.