I still remember the days ____ i stayed with grandpa in the countryside when I was a small child.

I still remember the days ____ i stayed with grandpa in the countryside when I was a small child.
为什么这里只能用when而不可以用on which
i stayed with grandpa in the countryside when I was a small child on the days 可以马？
在我是小孩子的那些天里，我和祖母生活在乡下 按中文来说是完全么问题的..不知道英文来说是咋样..