为什么一直报 ORA-00927错误（报错sql如下）

SQL code

update table set to_date(holiday_end,'yyyy-MM-dd') = to_date('2011-12-21','yyyy-MM-dd')where worker_id='10589'  and HOLIDAY_TYPE='旷 工'  and holiday_state='0' 

大家帮忙看下问题在哪？应该如何改，谢谢

[解决办法]
update table
set to_date(holiday_end,'yyyy-MM-dd') = '2011-12-21'
where worker_id='10589'
and HOLIDAY_TYPE='旷 工'
and holiday_state='0'