This is one of those events which happens on different days during the holiday season.
"Free Shipping" is an enticement to get you out there to buy more stuff from the retail establishments ...
I have been known to opt for the "freight paid" shipments where you buy merchandise in sufficient quantity to qualify for free shipping anyway.
I don't recall participating in a "free shipping" day per se.
Regardless, mid December is the timeframe of the official day. It just moves around a bit.
So if you want to buy some items and get a break on the freight charges this is the day for you !
Happy Free Shipping Day you thrift conscious shopper you. You may be able to wrangle a free shipping or "freight paid" shipment on your own if you're a wheeler dealer.