on children's day,boys and girls are all (dressed) in their dest clothes.为什么要加 ed?

on children's day,boys and girls are all (dressed) in their dest clothes.为什么要加 ed?

在儿童节,男孩和女孩们都穿上最好看的衣服
这里dressed相当于形容词,be dressed in 固定表达“穿着”.

dressed..... 要加ed....

可以理解为be done被动，be dressed被打扮成…
be dressed in some clothes 固定搭配，意思是盛装打扮。