I will never forget the days whiich I spent in the countryside last year.为什么不用when而用which?

which关系代词,定语从句中 缺乏主语、宾语时使用
这里 I spent the days spent缺了宾语 “度过那些日子”
where 关系副词,在成分基本完整时,选用
如:I will never forget the days which i lived in the countryside .