【英语】 that和when which where

【英语】 that和when which where
小弟英语还算可以
就拿when开刀
比如说有填空 1：Jhon will never forget the day ____ he joined the army.
2：Jhon will never forget the days____ he spent in Shanghai.
when 2是that
这两个有什么区别.
麻烦举个例子，句子的成分问题一直很纠结，