The day started before sunrise, on March 21, 1979, when Teacher Gu woke up and found his wife sobbing quietly into her blanket.
如果說 The day started before sunrise裏的day不是大家的day,是 Teacher Gu的那一天的話,即是Teacher Gu那一天的開始,就應該把“on”去掉,讓“March 21, 1979”變成“The day”的同位語。