区别就是两者意思是不一样具体的不同如下

onmonday中文意思是在星期一

例句：

Will he go on Monday or on Tuesday?

他是星期一走还是星期二走？

Would you come on Monday?

你星期一来好吗?

He'll arrive on Monday.

他将于星期一到达。

The store took stock on Monday.

那家商店每逢星期一进行盘点。

inmonday为错误搭配使用。