When you want to get name of day by using date in ABAP/4, you can apply function module "DATE_TO_DAY".

Example

*&---------------------------------------------------------------------*
*&      Form  get_name_of_day
*&---------------------------------------------------------------------*
FORM get_name_of_day USING    PAR_DATUM
                     CHANGING PAR_DAY_NAME.
  clear par_day_name.
  CALL FUNCTION ’DATE_TO_DAY’
    EXPORTING
      DATE          = par_datum
    IMPORTING
      WEEKDAY       = par_day_name
            .

ENDFORM.                    " get_name_of_day