That is a special day() we will never forget.

答案为D,如果有that也可以选that.
这句话是一个定语从句,that（可省略）引导的从句修饰day,怎样的一天呢?是我们永远不会忘记的一天.
we will never forget （the day）,day做的是宾语.
如果选择A,那么when引导的应该是一个时间状语从句.请注意下面第二个人给出的例句是错误的,因为那句并不是从句,只是一个连接词,译为“这时”.
引导时间状语从句的when相当于at/in/on +地方
例如I went to hongkong last Friday when the weather was beautiful.when在里面做时间状语,意思是the weather was beautiful last Friday.

e.g.
Alan was having dinner when he dad came home.

这个是定语从句，后半句是修饰day，引导词可以用which 、that 、或者省略