day做先行詞用on，which

days做先行詞的時候是可以用on which， 也可以用during which或者in which，根據days所表達的意思所決定。

I will never forget the days on which we went out to have a walk together.

day作先行詞也可以用that/which/when來引導定語從句，要看連線詞所代替的句子的意思

如果引導詞在定語從句中作狀語是可以的，這時on which 相當於when，比如: I still remember the day on which I met you.