Odds are, you probably have your holiday squared away. But procrastinators will be pleased to learn that there's one serious perk to putting off your gift shopping to the very last second: December 18 is Free Shipping Day , which means almost 1,000 retailers are offering free delivery on orders placed today with guaranteed arrival by Christmas. According to Kyle James, owner of Rather-Be-Shopping.com , this may be the best chance to save big on clothes and shoes.The annual day of shopping sans shipping costs, which started in December 2008, is actually based on a misconception about how close you can cut it with your holiday shopping. Luke Knowles, an entrepreneur who started the deals and coupon site FreeShipping.org , noticed a dip in online sales after December 10 and he connected the dots to realize that it stemmed from a concern that packages wouldn't arrive in time for Christmas if you ordered after the tenth — even though he knew retailers could fulfill those orders. So he brought shoppers together under the rallying cry of free shipping and a tradition was born."Many retailers will have a $50 or $75 threshold to qualify for free shipping during December, but then remove the minimum on Free Shipping Day," says James. "They do it because most shoppers have come to expect it and retailers typically see a big bump in online sales to close out the holiday shopping season."There are two rules for merchants that want to participate: "Brands must offer free shipping with no minimum order requirement and guarantee delivery by December 24 for orders placed on December 18," says Kendal Perez, a spokesperson for FreeShipping.org. All types of retailers can sign up — which allows for a mix of retailers from household names to independent sellers (even Etsy shops), she explains. The mid-December date is no accident, either."Free Shipping Day is the procrastinator’s shopping holiday, unlike Black Friday and Cyber Monday, so we schedule the event date about a week before Christmas," Perez says. It's always on a weekday, with the exact number switching around year to year, depending on what day of the week December 25 falls. It comes right on the heels of that other big shopping event in November, but James argues that Free Shipping Day is "way better than Black Friday in terms of coupons and deals," and also that it's "fairly comparable to Cyber Monday, which, historically, is an excellent day to shop online for fashion."You can truly buy anything on Free Shipping Day, but definitely look out for deals on clothing and accessories — whether it's for those stragglers on your gift list or simply as a pre-holiday treat for yourself."Many apparel and shoe websites will throw a coupon code for a percent or dollar off your purchase on top of the free shipping offer to entice shoppers," James notes. "We saw this a lot last year. Look for more of the same this year." Be warned, though: Delivery may be on retailers, but returns may not. "Be aware of the return policy of the online retailers, as you don't want to be hit with hefty return charges that will easily mitigate your savings," James says. "I always recommend knowing your sizes ahead of time to try and reduce this potential problem."Subscription programs like Amazon Prime and FreeShipping.com (no relation to FreeShipping.org, which runs Free Shipping Day) offer the service year-round to members for an annual fee. The latter launched back in 2002 — six years before Free Shipping Day became a thing. Though the site's subscribers pay a premium to get free shipping 365 days a year, there are still reasons for FreeShipping.com's members to fill their digital baskets with some goods. Last year, on December 18, the site offered 20% cash back on purchases (usually, users get 10%) on top of free shipping — and it worked: "Comparing our members’ daily activity from Thanksgiving through Christmas Eve, Free Shipping Day ranked fourth in overall clicks, second in member searches, fourth in total orders, fourth in total purchase amounts, and sixth in average order values," explains Tom Caporaso, CEO of Clarus Commerce , the e-commerce solutions firm that owns FreeShipping.com.Caporaso points to different studies that highlight just how compelling free shipping is to shoppers (and how beneficial it can be to brands ). "Consumers are much more willing to shop at retailers that offer free delivery and they’re willing to spend more on those purchases, so it pays off for retailers and shoppers alike," he notes. He says Free Shipping Day hits that sweet spot of giving shoppers what they want (no delivery fees, regardless of how much they spend) while also working the holiday time crunch (orders are guaranteed to be under the tree by Christmas).It seems like we have our work cut out for us...You know where to find us for the rest of the day.