Calendar Plus web part enables you to display events that are retrieved from external sources (Professional Edition).  This is done by creating a web service that implements a Calendar Provider interface. This interface defines a single method named GetData, that returns a DataSet object which contains a DataTable filled with event records.

This Calendar Provider web service source code example should be used as a template to build your web service.  For more information, please read the “Custom Calendar Provider” section in the Calendar Plus user guide.