一个意思,但如果说：在同一天,要用on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。