To change the calendar settings for a specific project:
- In the Work Plan view or Work Item tab, fighlight the project
- Right Click -> Properties
- Click Calendar tab
- Click on Schedule Settings link and set work days (then save!)
Once you've set your working days for a project, the project will automatically adjust using the new calendar. In the following example, I've set Saturday and Sunday as available work days for a specific project: http://screencast.com/t/cfVb6wm6Ot. As you will see, my dates will adjust if work can now fall on those new working days.