可以使用which和that,不能使用in which
与day搭配的介词是on,所以是on which,看下面的句子
I can't remember day on which I learned to walk
We often think of the days which we spent together on the island
两个句子里day充当的成分不同,所以连接词也不一样
I can't remember day on which I learned to walk 里的day充当时间状语
We often think of the days which we spent together on the island里的day作为宾语
学习更上一层楼!