Do you still remember the days ---- we spent together last summer?这个句子中的关系词要用关系代词that或which,因为在定语从句中缺少的是动词spend的宾语；
Yeah,I will never forget the days ---- we played togethe中的关系词要用关系副词when,因为在定语从句中,缺少的是动词play的状语.