Chocolate, chili and even salsa have all been feted in Harvard Square over the past several years, yet one of the culinary delicacies most associated with the Boston area-clam chowder-hasn't had its own celebration-that is, until now. On September 21 in front of 25 Brattle St. from 2-4pm, join fellow bivalve fans at the first annual Chowderfest, where close to a dozen Cambridge restaurants offer sample sizes of their creamiest clam concoctions, along with other soups and stews. This free festival also boasts a "mocktail" bar, hosted by Harvard Square favorite Grendel's Den. Thousands are expected to attend, so come early if you don't want to miss out on this inaugural event.