the night before. 过去的时间点以前已经搞定了。

A Festival for the Dead is held once a year in Japan.
This festival is a cheerful occasion, for on this day, the dead are said to return to their homes and they are welcomed by the living.
As they are expected to be hungry after their long journey, food is laid out for them.
Specially-made lanterns are hung outside each house to help the dead to find their way.
All night long, people dance and sing.
In the early morning, the food that had been laid out for the dead is thrown into a river or into the sea as it is considered unlucky for anyone living to eat it.
In towns that are near the sea, the tiny lanterns which had been hung in the streets the night before, are placed into the water when the festival is over.
Thousands of lanterns slowly drift out to sea guiding the dead on their return journey to the other world.
This is a moving spectacle, for crowds of people stand on the shore watching the lanterns drifting away until they can be seen no more.

我说一点自己的看法。其实这要从整个这件事的发展过程来看问题，整篇文章都是在介绍这个节日的过程，在悬挂灯摆放食物之后人们又做了很多事情，比如唱歌跳舞拉家常啊什么的等等，到了最后了才做流放灯笼投弃食物这件事，所以在这两个句子中所述的两件事之间其实是隔了许多的动作，有时较长的空距离，所以采用了过去完成式。如果改用现在完成式，时空距离就显得很近，在体现时空距离上没有过去完成式强。我是这么理解的，仅供参考，也许这个问题你早已解决了。