SQL & PL/SQL

Dear all,
I'm learning how TO_CHAR function and date format code work by using dual table, but I got some difficulties:

SQL> select to_char(sysdate,'dl') test from dual;

TEST
-----------------------------
Monday, December 20, 2010

SQL> select to_char(sysdate) test from dual;

TEST
---------
20-DEC-10

SQL> select sysdate test from dual;

TEST
---------
20-DEC-10

SQL> select to_char('20-DEC-10','dl') test from dual;
select to_char('20-DEC-10','dl') test from dual
               *
ERROR at line 1:
ORA-01722: invalid number

How can I pass an explicit date to TO_CHAR function?

Best regards,
Valerie