You know it’s almost basketball season when the entire Celtics team — including the front-office folks — puts on a suit and gets together for a fancy dinner. Typically, the C’s annual Shamrock Gala (benefiting the Boston Celtics Shamrock Foundation) is held at a downtown hotel. But Friday, displaying a bit fancy footwork, Celts officials held the gala at the TD Garden, transforming the arena into a groovy event space. Perennial all-stars Paul Pierce, Kevin Garnett, and Rajon Rondo were in the house along with new C’s Jason Terry, Courtney Lee, and Jared Sullinger, among others.