From the Brewers Association website:
Each year on the first Saturday in May, homebrewers unite non-brewing and brewing friends and family to celebrate National Homebrew Day, joining with thousands of homebrewers from around the world in brewing the same recipes and sharing a simultaneous toast at noon Central Time.
Before the event, participants that are planning BIG BREW events register their site on this web site. These registered sites help the American Homebrewers Association track how many participants celebrated the event. Event results will be posted on this page a few weeks after the event.
Visit the site for celebration ideas or to register a recipe.