World Hindi Day is celebrated on January 10 every year, marking the anniversary of first World Hindi Conference which was held in 1975. The first World Hindi Conference was inaugurated by the then prime minister Indira Gandhi. World Hindi Day was first observed on January 10, 2006.
World Hindi Day and National Hindi Diwas are completely different. National Hindi Diwas is celebrated every year on September 14. On that day in 1949, the constituent assembly adopted Hindi, written in Devanagari script, as the official language of the Union. While the focus of the World Hindi Day is to promote the language at the global stage.
Source: India Today