Calendar ClickedDate  
Calendar series have a ClickedDate method that returns the calendar day (date) under XY pixel coordinates:

var ClickedDate : TDateTime;

  if Series1.ClickedDate(x,y,ClickedDate) then
     Label1.Caption:='Date : ' + DateTimeToStr(ClickedDate)
  else
     Label1.Caption:='No date under XY';