World Post Day which falls on 9th October each year celebrates the establishment of the Universal Postal Union (UPU) in 1874. As one of the oldest international organizations in the world, the Union has been coordinating international postal policies in the last 140 years.
Cambodia joined the UPU in 1951. A set of 3 stamps was issued by Cambodia Post in 1997 to mark the death centenary of Heinrich von Stephan, founder of UPU. Here is the advertizing pamphlet of the issue:
Although no stamps have ever been issued by Cambodia for World Post Day, special cachets in red ink are sometimes prepared and handstamped on mail to commemorate the Day. At least 5 years have had the World Post Day cachets, here I illustrate cachets of 2005 and 2006: