So, technically the Monster Mash is a Halloween ritual, but Open Source Gallery in the South Slope (251 17th St at 5th Ave) is intent on defying tradition (and offering BBQ and free drinks in the process). To celebrate the opening of their latest exhibition (and, presumably, July 4th), Monsterbash, Open Source is throwing a BBQ this afternoon from 4-7pm, with food, games, crafts, music and free booze. The exhibition (which continues through August 15) involves a partnership between street artist Kylin O'Brien and kids at New York City public schools, and involves O'Brien creating murals based on children's drawings and descriptions of monsters. The first were unveiled last fall in Dumbo (like the one at right), but there are many more in the works. Click here for full details about the Monsterbash party and exhibition.