You could try something like this:
<DayPilot:MenuItem Text="Open" Action="JavaScript" JavaScript="window.open('Details.aspx?id={0}', 'windowname')" ></DayPilot:MenuItem>
Where {0} will be replaced by the event id.
Or:
<DayPilot:MenuItem Text="Open" Action="JavaScript" JavaScript="window.open('Details.aspx?id=' + e.value(), 'windowname')" ></DayPilot:MenuItem>
Where e holds the client-side DayPilot.Event object.