February 11th is National Foundation Day of Japan which is called Kenkokukinen-no-Hi in Japanese.
It is a national holiday in Japan to remind Japanese people of founding of the nation and encourage their love for the nation.
According to legend, Japan has been founded by its first Emperor Jinmu in 660 BC.
Emperor Jinmu has conquered Japan on lunar new year which falls on February 11th according to Gregorian calendar.
When Japan changed the lunar calendar to the Gregorian calendar in January 1873, Showa era, the day of the enthronement of Emperor Jinmu was designated as Empire day and was made a national holiday.