SQL> alter session set nls_date_format = 'dd.mm.yyyy';

Session altered.

SQL> with
  2  dates as
  3    (select trunc(sysdate, 'yyyy') + level - 1 datum
  4     from dual
  5     connect by level <= 20
  6    ),
  7  weeks as
  8    (select datum, to_number(to_char(datum, 'ww')) week
  9     from dates
 10    )
 11  select week,
 12         min(datum) min_date,
 13         max(datum) max_date
 14  from weeks
 15  group by week
 16  order by week;

      WEEK MIN_DATE   MAX_DATE
---------- ---------- ----------
         1 01.01.2015 07.01.2015
         2 08.01.2015 14.01.2015
         3 15.01.2015 20.01.2015

SQL>