The third Monday of July is a national holiday in Japan. Umi No Hi, also known as Ocean/Marine day was established as a national holiday in 1996. The purpose of this holiday is gratitude for the blessings of the oceans and to hope for the economic prosperity of maritime Japan. Prior to 2003 it was set on July 20, they moved it to third Monday in July so that people can have a long weekend. Prior to 1996 it was called Marine Memorial Day and was not a holiday. Prior to that it was the day that marked the return of Emperor Meiji from a boat trip to Hokkaido in 1876.
There are no traditions for this holiday. A lot of people decide to take a trip to the beach on this day though, aquariums organize special events, economic and cultural activities related to the sea are highlighted, and competitions/water sports' shows take place.
This is also the first day of summer break for school children in Japan.