when或on which
这是一个定语从句,先行词是the day,先行词放到从句中你看一下充当什么成分：
she first wore that pink dress (on that day) 是作时间状语,在那天她第一次穿那件粉色连衣裙
the day 用which指代是没错,但on这个介词不可省略的啊
因此作时间状语要用When,或者on which