Ok I'm a dbms_jobs idiot, I admit it

I 'd like to try and get the following to run every saturday at 6pm
and it's not working !
tks

steve

variable jobno number;
EXEC DBMS_JOB.SUBMIT(:jobno,'dbms_stats.gather_schema_stats(ownname=> ''SYSADM'' , cascade=> TRUE);'
, trunc(sysdate),''SATURDAY'') + 18/24, 'NEXT_DAY (TRUNC(sysdate),''SATURDAY'') +18/24');
commit;