Wikipedia depends on WP:V. If a reliable source mention there is a NMD in Japan, we have to add it. The quote from the book is "In Japan, there is a national masturbation day. Many websites boast about May being International Masturbation Month in which, among other activities, people are encouraged to get involved in masturbate-a-thons, various forms of self-loving, and mutual sexual play." This reference is checked by   Talk