定语从句I will never forget the day _____ we first met .用that 还是

when 正确.the day 是先行词,在定语从句中做时间状语填when
例如That was the day when i was born
That was the day that i spent in the city.that在定语从句中作宾语

B
定语从句中we first met.主谓不缺，因此不用that充当宾语（因为meet作相遇时是不及物动词），when做引导词，是定语从句的时间状语。

when

I will never forget the day ___when__ we first met .关系副词引导的限定性定语从句。
【还原一下】I will never forget the day.We first met on that day.
I will never forget the day __on which___ we first met .
我永远不会忘记我们初次相见的那一天。

when 做时间状语
这不是定语从句，

that