I'll never forget the day Mary left school on.

是什么从句？on the day 为什么分开？

这是定语从句

你先看这句
I will never forget the day（on which) when Mnary left school.
这里的when...是定语从句（不要理解成同位语，不能区分引导副词引导的定从和同位语从句可以另外查查）引导副词when/where/why在引导定语从句时，常常可以跟“介词 + 引导连词”交换使用，此结构可后置可前置，比如

I'll never forget the day when ( = on which ) we met for the first time .

The office where ( = in which ) you work is here .

The reason why ( = for which ) he did not come is quite clear .

比较常见的是前置，你这一句是后置了
可能是把on which当成介宾结构然后which省略了，以前我没注意过这时的引导词能不能省略，你可以问下老师