Today is the third Monday in July, which marks the holiday umi-no-hi, usually translated as Marine Day or Ocean Day or Sea Day. 
It was first celebrated as a national holiday in 1996 in remembrance of a sea-voyage the Meiji emperor took on July 20th 1876, where he went on a steam ship along the coast of Tohoku all the way up to Aomori prefecture and even Hokkaido before returning to Yokohama.
Before that, it was simply called Marine Memorial Day and even today it is meant to consider the importance the ocean has on an island country like Japan (which is one of the major harvesters of seafood and fish, by the way).
Since 2003, it has been moved in to the third Monday of July in accordance with the “Happy Monday” system.