![day做先行词用on which]( "day做先行词用on which")
days做先行词的时候是可以用on which， 也可以用during which或者in which，根据days所表达的意思所决定。
I will never forget the days on which we went out to have a walk together.
day作先行词也可以用that/which/when来引导定语从句，要看连接词所代替的句子的意思
如果引导词在定语从句中作状语是可以的，这时on which 相当于when，比如: I still remember the day on which I met you.
