Weekday (day of week)
to_char(some_date,'D') returns a number indicating the weekday. However, for a given date, this number returned varies with NLS_TERRITORY:
SQL> alter session set nls_territory='UNITED KINGDOM';
SQL> select to_char(sysdate,'D') from dual
SQL> alter session set nls_territory='AMERICA';
SQL> select to_char(sysdate,'D') from dual;
I executed these select statements on a thursday. So, in America, Thursday seems to be the 5th day in the week while it is the 4th in the UK.