One day, it'll happen. You'll see.有朝一日，它会发生。你会看到的。

People may one day [=someday] be able to take vacations to the moon.或许将来有一天人们能够到月球去度假。

I went to her house one day and had lunch with her.有一天我去了她家，还和她共进了午餐。

One day, we had a terrible argument.有一天，我们发生了激烈争吵。