Really, it is!
As this article on http://www.nohoartsdistrict.com says:
Hey North Hollywood! Did you know that Sunday, July 14 is International Nude Day? This holiday originated in New Zealand when former rugby star and TV presenter Marc Ellis dared people to streak in front of Prime Minister Helen Clark. I guess there are many fans of nudity because in the past five years more than 30 countries now celebrate International Nude Day.
So nudists, naturists, nekkid people, whatever label you like, get out and be naked with a vengeance coming Sunday.