July 18 was chosen because that is the feast day of St. Anou of Metz (a.k.a. St. Arnold), the best known patron saint of brewers.
How to celebrate? Well, drink beer for one. Profile your favorite brewmaster. Hold an event. Hug your favorite brewer. Petition Hallmark to create greeting cards for the occasion. Whatever you do, tell the world about it. For more information, please check out the official International Brewers Day website.