Code of trigger not working

Can anyone help me in finding whay the below code is not working. It says "invalid character"
It is a trigger on table teacher that restricts from updating and deleting of the records on weekends.

Code:

CREATE OR REPLACE TRIGGER NOT_WORKING _HOUR 
BEFORE INSERT OR DELETE OR UPDATE ON TEACHER 

BEGIN 
IF (TO_CHAR(SYSDATE,'DY') IN ('SAT','SUN')) THEN 
DBMS_OUTPUT.PUT_LINE('SORRY! YOU CANNOT UPDATE OR DELETE ON WEEKENDS'); 
END IF; 
END