ActivateDate does just that - it makes the specified date to be the ActiveDate but doesn't manipulate the selection. Currently there is no command that will activate and select a date (and signal that the selection is done when it is used in a dropdown like in xamDateTimeEditor). Perhaps we can add another command. I'll see about getting an issue written up about this.

Ok I do have the Command executing now - It's based on a Vm bound souce.  But the functionality does not match when using MonthCalendarCommands.Today.
MonthCalendarCommands.Today activates the Calendar to the Month where the date is set and it also selects (highlights) the Date then closes the Calendar.

Using MonthCalendarCommands.ActivateDate only activates the Date I send as CommandParmater but does not select (highlight it) it NOR close the Calendar.

I still need it to mimic .Today command.

See my updated attached XAML

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>