World Post Day is celebrated each year on 9 October, the anniversary of the establishment of the Universal Postal Union in 1874 in the Swiss Capital, Bern. The theme selected for the 2018 competition is: “Imagine you are a letter travelling through time. What message do you wish to convey to your readers?”
It was declared World Post Day by the Universal Postal Union (UPU) Congress held in Tokyo, Japan in 1969. Since then, countries across the world participate annually in the celebrations.
Source- The United Nations