WITH t AS
(
SELECT TO_DATE('201212', 'yyyymm') + LEVEL - 1 dt
FROM dual
CONNECT BY LEVEL <= 151
)
SELECT dt
     , TO_CHAR(TRUNC(dt + 3, 'd'), 'yyyymm')
       || '-' ||
       DENSE_RANK() OVER(
       PARTITION BY TO_CHAR(TRUNC(dt + 3, 'd'), 'yyyymm')
       ORDER BY TRUNC(dt + 3, 'd')
       ) ym_w
  FROM t
 ORDER BY dt
;