One day
Definition 1 of 3
Pro Tip 1/3Future Ambitions
Used to express hopes or dreams that someone believes will happen. 
One day, I will travel around the world.
Pro Tip 2/3Not Scheduled
Indicates a future event without a specific time planned. 
One day we should go hiking together.
Pro Tip 3/3Narrative Twist
Adds a sense of mystery or change in storytelling. 
One day, she discovered the secret door.