具体到某一天（星期）,或某一天的早上/下午/晚上用ON,或名词前有形容词时也是用ON,如
on Saturday
on the morning of December 18th
on a cold winter morning
什么情况下用 on
具体到某一天（星期）,或某一天的早上/下午/晚上用ON,或名词前有形容词时也是用ON,如
on Saturday
on the morning of December 18th
on a cold winter morning