World Post Day is celebrated annually on 9 October to mark the establishment of the Universal Postal Union (UPU) in 1874 in Berne, capital city of Switzerland. In 1969, it was declared as World Post Day by the Universal Postal Union in Tokyo, Japan.

The motive behind celebrating the World Post Day is to create awareness of the role played by postal sector in people's everyday lives. The celebration encourages member countries to participate in activities aimed at spreading a broader awareness of their postal service's role among the citizens and how it helps them have a better life.

More than 150 countries celebrate World Post Day every year in various ways. In certain countries, it is observed as a working holiday, while a number of countries use the event to introduce or promote new postal products and services. Some postal services also reward their employees for good service on this day.

As the world celebrates World Post Day, here is a list of 10 of the rarest and most valuable stamps ever printed in the history of posts.

1. One Penny Black – world's first stamp (1840)

One Penny Black stamp was first issued by the British government in 1840 and features the bust of England's Queen Victoria.

The early printing of these stamps were attached to each other and had to be cut with scissors or blade. Unused Penny Black stamps are a rare sight today.

Currently, the market value of an unused One Penny Black stamp is approximately $3, 000.

2. Hawaii Missionary Stamp (1851)

Hawaii Missionary stamp is one of the most expensive stamps in the world. It was one of the first and oldest stamps made in Hawaii.

The stamp was used by American missionaries who came to the Hawaiian islands in the 19th century.

A set of eight unused Hawaiian Missionary stamps can cost up to $100, 000.

3. British Guiana 1-cent Magenta Stamp (1856)

This stamp is considered as the most rare and valuable stamp that ever existed. In an auction in 1970, it was sold for $280, 000

Currently, the value of this stamp is estimated to be over a million dollars and there is only one original specimen left.

Several fake British Guiana stamps resurfaced through the years.

4. The New Brunswick stamp "Connell's Folly" stamps (1859)

The postmaster, General Charles Connell, used his own picture on the 5 cent stamp that was the standard value in the Canadian colony.

However, the public opposed the act for Connell's political propaganda.

This opposition made this rare Canadian stamp controversial.

5. The China "Special Delivery" Issue stamp (1913-1914)

This Chinese stamp is one of the world's largest stamps with five different variants.

The size of each stamp measures 247mm by 65 mm. 

6. The U.S. "Inverted Airmail" postage stamp (1918)

An inverted Airmail stamp has a value of $42, 500.

This stamp is also known as Jenny stamp as it has a design of a Curtiss JN-4 airplane that was mistakenly printed upside-down. 

7. The U.S. "Dag Hammarskjold Error" stamps (1962)

The stamps were bought by two collectors; one from New Jersey and another from Ohio. The stamps bought by them had the yellow background inverted.

When the Post Office department found about the error, they reissued the stamps with the error.

These stamps are also known as the Day's Follies after Postermaster Edward Day.  

8. Bluenose stamp (1929)

This Canadian stamp is considered to be one of the most beautiful and rare stamps. it has a picture of Bluenose, a schooner (racing ship) from Nova Scotia. 

it has an estimated value of CAN $700. 

9. Uganda Cowries stamps (1895)

Also known by the name of Uganda Missionaries, this is one of the first Ugandan stamps. 

Due to lack of any printing press at that time, the first stamps in Uganda were typed on a typewriter.

A pair of these stamps cost £2, 000, 00.

10. Inverted Head Four Annas stamp (1854)

This Indian stamp was created using a blue and white dye.  It has an upside-down effigy printed with multicolour.

Source: 2-Clicks Stamps