have two weeks' vacation为什么在two前不加a?

have two weeks' vacation为什么在two前不加a? we have two weeks vacation for Christmas这句为什么不加will? I will spent two weeks taking a vacation in QinDao.() I will srend two weeks taking a vacation in Qingdao. After the exam,we'll have﹍﹎holiday.A two weeks B two-weeks C Lin Tao ____(have)a party in two weeks two weeks' time为什么要有“ ’” a leave of two weeks=two weeks leave吗 I will spend two weeks taking a vacation in Qing.(改为同义句) we'll be away for two weeks becaude we'll have a( ) A.two-we We have computer lessons ()A.once two weeks B.twice two week two weeks later 是用在过去时?为什么?