Traditional, homemade Christmas cookies lie on plates in a household on Dec. 21, 2010 in Berlin. Getty Images

As if you needed more calories between Thanksgiving and Christmas, Sunday is National Cookie Day.

The delicious holiday is an informal one, but it's well-known — it's even mentioned on the National Restaurant Association's official website. Its history is unclear, though notes that the Cookie Monster from Sesame Street once started a similar tradition.

If you're hoping to celebrate Sunday, check out these freebies, restaurant promotions and product deals:

Score a free small cookie when you buy any entree at Schlotzsky's locations Sunday. You can upgrade your cookie to a bigger size for $1 and also tweet @schlotzskys with a cookie emoji to get a coupon. The company will be handing out cookies at United Service Organization centers, as well.

Tag Hot Box Cookies in a social media post between noon and 6 p.m. local time Sunday to get two free chocolate chip cookies.

Stop by Pei Wei Sunday to get a free cookie with any entree or kids meal purchase.

You can get chocolate chip cookies for $0.25 all week at Whole Foods Market. Bring a friend — there's a limit of three per person.

Get a free cookie with any purchase at Insomnia Cookies locations Sunday.

Sing a cookie-themed version of "Jingle Bells" to the cashier at any ROCS Subway or Fresh Kitchen restaurant in West Virginia to get a free cookie.

Good Dee's sugar-free cookie mixes are $2 off on Amazon Sunday.

Use the promo code JOLLY to get free ground shipping on Mrs. Fields online orders through Dec. 23.

Head to Sweetology in St. Louis to get a free cookie with any purchase over $10 Sunday.

Snag a free cookie with every drink you buy at The Parlour cafe in the Charmant Hotel in La Crosse, Wisconsin.

The Cravory in San Diego, California, is discounting their signature and holiday cookies to $1 Sunday.

Get a free sample of the new Hershey's Cookie Layer Crunch Bars in Las Vegas, Chicago and Philadelphia Sunday.