CALL FUNCTION 'RH_GET_DATE_DAYNAME'   
       EXPORTING
               LANGU = SY-LANGU
               DATE  = PLANDATE         "입력된 변수임 
               CHECK = ' '
       IMPORTING
               DAYNR  = LV_DAYNR
               DAYTXT = LV_DAYTXT.

  -> LV_DAYNR =  '7'  (일요일)