Triggers

hi, i'm having trouble writing a couple of triggers for two constraints in Oracle.

the constraints are:
* changes to STAFF records should not be allowed on weekends
* a STAFF member's Date_left should not be before the Date_started

the STAFF table is as follows..

Staff_id varchar2(10) NOT NULL,
Name varchar2(35) NOT NULL,
address VARCHAR2(50) NOT NULL,
teleno varchar2(10) NOT NULL,
NIno varchar2(10) NOT NULL,
Notes VARCHAR2(500) NOT NULL,
Department Varchar2(20)
Staff_responsibility Varchar2(20)
Date_started Date,
Date_left Date,
CONSTRAINT STAFF_PK
PRIMARY KEY (Staff_id)


any help writing these two triggers is much appreciated.