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 2–4 p.m., 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.