what is waning day?

The period nearing the end of an event or a person's office

某事件、時代或任期的最尾時段

e.g. the waning days of the British Empire
the waning days of the president's office
the waning days of the Second World War

a gradual decrease in magnitude or extent (of the moon) pertaining to the period during which the visible surface of the moon decreases