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

------解决方案--------------------
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'