I'll never forget the day _____ I first met my best friend.用that还是when？急！

用when,引导从句修饰名词days。

I'll never forget the day when I ...

其他例子：
-I met him the day when I first came to the school.
-The days when the Chinese suffered from deadly infectious diseases have all gone.
-I still remember the happy days when I was in Europe.

我觉得应该是when .更强调the day

the day 是一个时间，所以用when 引导，意思是当...的时候。

应该是that.或者不填

that