The image of Uncle Sam is a personification of the United States of America as a nation and government. Today it is widespread and instantly recognisable. In honor of this general image "Uncle Sam Day" was officially declared in 1989 on the birthday of Samual Wilson - 13 September.

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Uncle Sam

On this day in 1813, the United States gets its nickname - Uncle Sam.

During the War of 1813 Samuel Wilson (1766-1854), a meat packer from Troy, New York, supplied barrels of beef to the United States Army. On the barrels, he stamped “U.S.” for the United States. The soldiers, who linked the initials to him, started to refer to these barrels as “Uncle Sam’s.”

In the late 1860s and 1870s, political cartoonist Thomas Nast (1840-1902), revive the “Uncle Sam” name again when he added an image to it.

Nast evolves the image over time and eventually added a white beard and stars-and-stripes suit which are associated with “Uncle Sam” today.

It was the artist James Montgomery Flagg(1877-1960) who created the most famous image of Uncle Sam.

Flagg added the tall top hat and blue jacket while pointing straight ahead at the viewer. During World War I, the words “I Want You For The U.S. Army” was added and used as a recruiting poster.

The U.S. Congress acknowledge Samuel Wilson in September 1961 ​as “the progenitor of America’s national symbol of Uncle Sam”.

Sources:
http://americanhistory.si.edu/blog/2013/09/uncle-sam-the-man-and-the-meme-the-origins-of-uncle-sam.html
https://global.britannica.com/topic/Uncle-Sam
http://www.history.com/this-day-in-history/united-states-nicknamed-uncle-sam