Never.

Good-night? ah! no; the hour is ill
Which severs those it should unite;
Let us remain together still.
Then it will be good night.

How can I call the lone night good,
Though thy sweet wishes wing its flight?
Be it not said, thought, understood,
Then it will be good night.

To hearts which near each other move
From evening close to morning light,
The night is good; because, my love
They never say good-night.

Percy Bysshe Shelley