let's make it at 3 o'clock on saturday afternoon ,为什么要用介词it?

it 指约定
这里指约定在,时候
问题补充：
sorry,字打错了,应该改为：为什么要用介词at,另一本书上说不应该用那个介词,所以希望各位帮忙解决.
其实,用at还常见些,at可要可不要,因为,make 既可接名词,又可接介词短语做宾补,见例句：
We can make it at 8∶ 00.
我们可以定在8∶00.
Do you think you can make it at...
你看能不能定在……呢?
I'm afraid I can't make it at 5 o'clock.
明天下午5：00我恐怕不行.
.I'm afraid he can't make it at 4:00.Would you make it another time?
恐怕4点他不行,你能改个时间吗?
That's o.k.then let's make it three o'clock at my office.
很好.那咱们约定3点在我办公室见.