你是否分得清back in the day和back in the days

back in the day

过去；从前；旧时

in the past

My dad's always talking about how great everything was back in the day.

我爸爸总是讲过去的一切如何如何的好。

back in the days

在过去的某个时候

at a particular time in the past

I was a fan back in the days when the band wasn't yet famous.

在这个乐队还没出名的时候，我就是他们的歌迷。