He said he would never forget the day ___he spent with his girlfriend in the country

He said he would never forget the day ___he spent with his girlfriend in the country
A.that　 B.when
C.how 　D.where
为什么不能用when.
为什么这些句子都能用when
He will never forget the day ___when__ he failed in the exam.
I will never forget the day __when__we met for the first time