We are going to play basketball----- saturday afternoon

on
in the morning“在上午”
in the afternoon“在下午”
in the evening “在傍晚/晚上”
at night“在夜里”
但是具体指某一天或某一天的上午/下午/晚上,用介词on.
It was a morning on Tuesday.A new colleague came to work in my office.
那是一个周二的上午,办公室里来了一个新的同事.
It happened on Sunday morning.
那件事发生在星期日早晨.
He put on a thick sweater since it was cold in the morning.
早晨很冷,他穿上了一件厚毛衣.
I get up early in the morning.
早晨我起床很早.