From the World Photography Day website:
World Photography Day is an annual, worldwide celebration of the art, craft, science and history of photography.
https://www.worldphotographyday.com/
While participants are encouraged to share their best photos on social media today, I try to share my best photos year round, on this website, for you to enjoy and learn from. As to why August 19th is the day, we have to go back to 1839 for the answer:
Daguerre (1787–1851) and the Invention of Photography
#WorldPhotographyDay
