You can add/change the schedule of a project by going to 'Edit Project' > 'Setting' > 'Schedule' and choosing a schedule from the drop-down. You will need to set up a schedule that includes weekends if you would like to schedule work during the weekends. You do this by going to 'Setup' > 'Schedules.' Here you can setup a new schedule by clicking the 'New Schedule' button. Here you can name the schedule, assign it to groups, outline the the days and hours available on the schedule, and add any exceptions for the schedule.