1.by day意为“在白天”，与 by night（在夜间）相对。

They sleep by day and hunt by night.

他们白天睡觉，夜晚捕猎。

Nocturnal animals sleep by day and hunt by night.

夜间活动的动物白天睡觉晚上猎食。

The weather did not help; hot by day, cold at night.

天气也不肯帮忙：白天很热，晚上很冷。

The city, drab and dour by day, is transformed at night.

这座城市白天死气沉沉、单调乏味，晚上就完全变了样。

Most of us work by day，and only a few work by night. 我们大多数人上白班，只有少数人上夜班。

2.by the day意为“按天计算”。同样的表示法还有：by the week（按周），by the month（按月），by the year（按年）。

We rented the car by the day.

我们按日租用汽车。

The company hires out machinery by the day.

那公司按日计费出租机械设备。

Cleaning women in big cities get paid by the day.

大城市里的女仆工资按日计。

They are paid by the day while we are paid by the week. 他们的工资按天发，而我们的工资按周发。

These cleaners got paid by the month.

清洁工人按月拿工资。

注意：

Their aim is equal numbers of male and female MPs by the year 2000.

他们的目标是2000年前使国会议员中男女人数相等。

If current trends don't bend, Russia's population will be about the size of Yemen's by the year 2050 ．

如果目前的趋势不改变,到2050年,俄罗斯的人口将达到也门的人口规模。

But by the year 2020 business computing will have changed beyond recognition.

但是到了2020年，商业计算会变得面目全非。