How to trigger workflow when Calender list item reaches due date in Sharepoint?

There is nothing out of the box that I can think of that will satisfy this requirement.  The SharePoint events that are available to start a workflow include events such as Item Created, Item Modified, Item Checked-In, Item Checked-Out, etc.   You might have to create a custom assembly that reads the items in the list (calendar) and starts a K2 process if System.DateTime.Now == the due date of the calendar item.  Wouldn't be difficult, but you'll have to write and maintain the code.

Eric Eichler
5th Method Consulting
http://www.5thmethod.com
K2 Partner