select to_char(sysdate,'day') from dual

--wednesday

select substr(to_char(to_date('2015-01-20','yyyy-mm-dd'),'day','NLS_DATE_LANGUAGE=''SIMPLIFIED CHINESE'''),-1) from dual;  

--- 二

select substr(to_char(sysdate,'day','NLS_DATE_LANGUAGE=''SIMPLIFIED CHINESE'''),-1) from dual;  

---三