This 51-foot peanut butter and jelly sandwich is cut up for consumption on National Peanut Butter and Jelly Day in Escondido, California on April 2, 2014. Reuters

Thursday is National Sandwich Day, an unofficial opportunity for Americans to celebrate one of their favorite foods. Sandwiches are seriously popular in the United States: A 2014 study found that about half of citizens over age 20 eat one every day, according to Smithsonian Magazine.

However, the food wasn't an overnight sensation — it has a storied history. Although there are several legends about how the dish was invented, the most widely known one tells the tale of John Montagu, the fourth Earl of Sandwich.

The myth, which started in a 1700s Pierre Jean Grosley travel book called "Tour to London," explains that the earl liked to eat meat and bread while gambling, according to Montagu, who liked to talk while gambling, would request to be served two slices of bread with meat in between them. Eventually, his gambling buddies began to order the same thing, asking for "the same as Sandwich."

No matter what story you believe, Thursday is a chance to indulge — especially given the promotions several restaurants are running. Below are a few places that are offering freebies during National Sandwich Day:

Subway is offering customers a free sub with the purchase of any sub and drink. The promotion, which is called "The Good Deed Feed," will give one meal to the nonprofit Feeding America with each purchase.

Arby’s will give customers a Free Roast Beef Classic for signing up to receive promotional emails. But beware: A drink purchase is required to get the free sandwich.

Customers can win a year of free Potbelly by entering a contest held by the sandwich shop. On National Sandwich Day, go into Potbelly, take a photo of your sandwich and post it to Instagram with the hashtags #NationalSandwichDay and #PBEntry. Contestants must also tag the establishment at @PotbellySandwichShop. The sandwich with the most likes in 24 hours will win free Potbelly for a year, while the 10 runners-up will walk away with coupons for other menu items.

Blimpie’s is offering a free regular sub with the purchase of a regular sub and drink if customers sign up to join their e-club.

Schlotzsky's will celebrate National Sandwich day by giving customers a free medium sandwich with the purchase of a medium sandwich of equal or greater value. "We’re a brand that was originally built on a sandwich, so National Sandwich Day is the perfect way to reward our guests, and spotlight our growing sandwich lineup," Schlotzsky’s president Kelly Roddy said in a news release. "We hope to help Schlotzsky’s lovers across the country get their sandwich fix by offering our guests their favorite one for free."

Capriotti’s is offering customers the opportunity to win free nine-inch subs for a year. Customers can enter by mail for free or go into one of the stores, complete a minimum purchase of $4.99 and register on the CAPAddicts Rewards App.

By downloading the BK app from Burger King, customers can receive a free sandwich with the purchase of a Whopper. The sandwich must be of equal or lesser value than the company’s signature burger.