Their birthday are in the same day!哪里错了.

on the same day
同一天,不是用in
in the same day和on the same day有什么区别
英汉词典上给出的例句：
On the same day there was another straw in the ill wind.
同一天,这阵恶风又带来一种不吉之兆.
They were both born on the same day and in the same town.
他们俩同一天在同一个城市里出生.参考资料：句酷