marine day
From Wikipedia:
"Marine Day (海の日, Umi no Hi), also known as 'Ocean Day' or 'Sea Day', is a public holiday in Japan usually celebrated on the third Monday in July. The purpose of the holiday is to give thanks for the ocean's bounty and to consider the importance of the ocean to Japan as a maritime nation. Many people take advantage of the holiday and summer weather to take a beach trip. Other ocean-related festivities are observed as well."
On this day, various Japanese artists will post an image of characters in their swimsuits to celebrate the holiday. Apply this tag when Marine Day is mentioned by the artist, either within the image or in the commentary.