所谓先行词,就是名词,代词,或名词短语,所以,本句的先行词是the first few days .
本句when +in which .
先行词在从句中作状语时,用关系副词when；作主语,宾语,表语是,用关系代词which/that.
例句：I still remember the day when I joined the Army. the day 在从句中作时间状语,故用when
I still rmember the day which I spent in the countryside. the day 在从句中作spent 的宾语,故用which