in the days 还是on the days?

题目

in the days 还是on the days?
1 The days on which black people in the country could not travel without a pass are gone forever.
2 I will never forget the days in which we worked together.
为什么第一个用on which 第二个用 in which

答案

on the days 表示特指,也就是说话人都知道的 而第一句说的是在这个国家里黑人旅行需要通行证的日子已经过去了,是特指那些需要通行证的日子
in the days 指一段时间
和in the morning与 on the morning是一样的道理