In the XAml attachment, it should have the Command property on the Button as:
{x:Static igEditors:MonthCalendarCommands.ActivateDate}
Its probably because you are passing a string as the command parameter and not a date. Try setting the CommandParameter to a DateTime. e.g.
<Setter Property="CommandParameter"> <Setter.Value> <sys:DateTime>12.13.2012</sys:DateTime> </Setter.Value></Setter>