倒计时几天的英文表达是 "countdown (to) (the) (number of) days". 来源是倒计时（countdown）这个词，其本意为数数，计算剩余时间。因此，可以在其后加上需要倒计时的天数。

例句：

There are only three days left until the wedding, so we're in a countdown to the big day.

We've started a countdown to the number of days until our vacation.

It's the last week of school, and we're in a countdown to the last day.

中文翻译：

婚礼只剩下三天了，所以我们正在倒计时。

我们已经开始倒计时，计算出度假的天数。

这是学校的最后一周，我们正在倒计时最后一天。