英语专业回答：这道题的答案是when,解析：day后面是一个定语从句来修饰先行词day,而定语从句中缺少状语,所以用when,这道题还可以这样做I will never forget the day on whic I was born 或者I will never forget the day which I was born on,不过原题born后没单词,可以排除这种情况.I will never forget the day on whic I was born这里的on which=when