How do you top the 100th anniversary of the invention of Marshmallow Fluff in Somerville’s Union Square that was marked in grand style at the What the Fluff? (flufffestival.com) festival last year? By adding a supernatural twist! In honor of this 13th annual affair, the organizers have themed this year’s party “Fluff the 13th” and introduced imagery of black cats, fortune tellers and other magical mysticism to the usual mix of musical performances, food, cooking and costume contests, Fluff-inspired games (think Fluff jousting, Fluff bowling, a marshmallow toss and the like) and other family fun. Head down to the streets of Union Square September 22 from 3–7 p.m. to get in on this sweet, sticky and silly celebration for all ages.