S.F. loves a good flash mob (don't you? we don't) and today a mob of several hundred, if not more, are expected to irritate and confuse Union Square tourists around noon. The event is called One Dance, they've been doing it for a couple of years, and it kicks off National Dance Week. Participants are asked to "perform" a three-minute "choreographed dance piece" between noon and 1 p.m., on the Square. The coming week will also feature over 400 free dance events around the Bay Area.