A lot of online retailers and brands are in on it
That includes Guess, J.C. Penney, Target, Lululemon, The North Face, GameStop and more than 1,100 others ranging from the high-end to the cheap stuff. Basically, if a retail chain has any notable presence in United States malls, this will save you a trip to those nexuses of holiday hell. Notably absent online retailers include Amazon, Walmart, and Jet.
Apparently there are additional deals -- not just free shipping
But don't get too excited. These won't be Black Friday/Cyber Monday-level deals, and these will be offered case by case by the individual retailers. Also: there's no official spending minimum for the Free Shipping Day. Independent of Free Shipping Day, FatWallet has a good list of holiday deals currently active (their list includes sites like Amazon, Walmart, and Jet, so you're in luck).