But when men began to build cities, they wanted to have a special day on which to trade, a market day.
【正确答案】不过当人类开始建造城市时，他们想有一个做交易的特殊日子，这就是市场日。
【答案解析】在day这个名词后有on which to trade作后置定语，因此需要提到day的前面来翻译；a market day是作为a special day的同位语来出现。