南昌站到南昌工学院:The letter I had been looking forward _____at last.

The letter I had been looking forward _____at last. After I sent the letter asking for a job, I had nothing to do but await the answer. He had already posted the letter ____five yesterday. By the time i was seven ,i had 1 ____ the classroom,I found him writing a letter. At last we got the letter we had long been expecting. “He had already posted the letter by five yesterday.”为什么要用介词"by"? I had the edge though是什么意思呢？ 谢谢高手指点! I had worked deep into the night,so I hated to get up the next morning. I had meant __you __to the host at the party,but I forgot later.