Rich country-club types famously used to wear seersucker suits and white bucks to the office between Memorial and Labor day - so the attorneys who worked for prestige firms dressed in this way during the summer, while the schlubs who worked for lesser firms couldn't afford seasonal wardrobes and wore normal suits year-round.
Well that makes sense...it should have occurred to me. Funny though that this "hip" writer, steeped in au courant gossip, would use an apparently antiquated term.