Remove weekends from Burndown in Board View (TFS 2012 - Scrum 2.0)
UPDATE: The burndown excludes the weekends on Visual Studio Online. Later this year, this feature ships with TFS 2013 Update 2 for on-prem customers.
See the announcement at http://www.visualstudio.com/news/2014-jan-22-vso.
If you want to configure the working days of your team, you can click on the gear button in the top right corner which will open the admin area in a new tab. Click on your team, and choose the “settings” tab.
The TFS team on uservoice explained recently that they started working on this feature request Burndown chart should exclude non-work days.