The 24-hours calendar in Project already includes weekends as working time. It is 24 hours per day 7 days per week 365 days per year.

A 16 hour task will still show as 2 days duration because although you have assigned the 24 hours calendar, the definition of "day" = 8 hours. The start and finish date/times will be correct.

Add the Task Calendar field to a task view to quickly set calendars.