没有at morning之说，只有on morning. 固定搭配 on Sunday morning，表示 时间，比如 I usually go shopping on Sunday morning. 介词at 一般指在某个地方，或者指某一特定时间段。例句： I was cooking by the time you was knocking at the door.(at the door)
I went to post office at 4pm yesterday afternoon.(at 4pm)
