Marine Day, known as 'Umi no Hi' in Japan, is a national holiday that is annually celebrated on the third Monday of July. This holiday was initially established in 1996 as a day to honor and give thanks to the ocean's bounty and its significance to Japan as an island nation. It was also to commemorate the return of Emperor Meiji from a sea voyage in 1876 in the Meiji-Maru, an iron steamship. It was Japan's first official national holiday that fell during the summer season.

On Marine Day, many families take advantage of the holiday and warm summer weather to take a trip to the beach. It's a day where different Japanese communities organize special events. People engage in various ocean-related activities like releasing floating lanterns into the sea and cleaning up beaches. It's a day of not only embracing the sea's elegance and its gifts but also raising awareness about the protection and sustainability of the marine environment.

Though it's a relatively new holiday in Japan’s long history, Marine Day has quickly become a cherished opportunity for citizens to reflect on their relationship with the sea and to enjoy a day off in the height of summer. It doesn't have specific traditions or customs attached to it as many other Japanese holidays do. But it nevertheless holds a special place in the hearts of the Japanese people, for whom the sea has always been an essential part of life and culture.