WITH t AS
(
SELECT sysdate dt FROM dual
UNION ALL SELECT TO_DATE('20201221', 'yyyymmdd') FROM dual  -- 2020_52
UNION ALL SELECT TO_DATE('20201228', 'yyyymmdd') FROM dual  -- 2020_53
UNION ALL SELECT TO_DATE('20210104', 'yyyymmdd') FROM dual  -- 2021_01
)
SELECT dt
     , iyyy
     , iw
     , TO_CHAR(iyyy - 1) iyyy_pre
     , DECODE(TO_CHAR(NEXT_DAY(TO_DATE((iyyy-1)||'01', 'yyyymm') - 1, 5) - 3 + (iw-1)*7, 'iw')
       , iw, iw, iw-1) iw_pre
  FROM (SELECT dt
             , TO_CHAR(dt, 'iyyy') iyyy
             , TO_CHAR(dt, 'iw') iw
          FROM t
        )
;