这句话可以改成 I'll never forget the day. (the day 是forget的宾语)
改称倒装句强调“the day",就是 That's the day which I'll never forget. 所以 which 是关系代词指the day.
虚拟语气?
你是新手吧,which引导的句子是前面the day的定语从句,而从句也是一句子,句子成分齐全,在这里which指的是the day 我将永远忘不了那一天
which 是作为定语从句的一部分的,整个从句修饰the day,而which在从句中代指the day做forget的宾语。