You probably know that today is National Doughnut Day. But do you know the story behind the country's tastiest not-really-a-holiday? It has to do with why the 2017 version of National Doughnut Day should taste a little bit sweeter.

First the background, in case you haven't been on social media all morning: National Doughnut Day is the first Friday of June each year, and it's best known now as a marketing gimmick--a chance to get free doughnuts at Krispy Kreme, Dunkin' Donuts, and other places. (Here's a pretty good list.)

However, there's a surprising and inspiring story behind the celebration. Today is actually the 79th annual National Doughnut Day, which in turn commemorated an act of caring that the United States engaged in 100 years ago.

Just after the U.S. sent troops to France in 1917, the Salvation Army sent volunteers, too, to find ways to provide support for American servicemen. This was the first large-scale deployment of U.S. troops overseas, and one of the most efficient ideas the Salvation Army came up with was to assemble huts near U.S. training centers where they could provide some of the comforts of home.

Those comforts included baked good, but of course there were challenges for volunteers trying to run entire bakeries in a hut near a war zone. So, a pair of Salvation Army ensigns, named Helen Purviance and Margaret Sheldon, are credited with coming up with a solution: frying doughnuts, which they figured out how to make quickly and at scale.

"I was literally on my knees when those first doughnuts were fried, seven at a time, in a small frypan," Purviance later recalled.

They had a hit on their hands--in a diary Sheldon recalled making 700 doughnuts a day herself--and the image of volunteers providing doughnuts to homesick American soldiers caught on back home.

Twenty years after the war, in the thick of the Great Depression, the Salvation Army seized on the idea of commemorating the doughnut effort, and started National Doughnut Day as a fundraiser. It caught on, and as doughnut themed restaurant chains sprung up later in the 20th century, they seized on it as well.

There's even a great story about how U.S. prisoners of war in Vietnam tricked their captors into serving them sticky buns by convincing them that National Doughnut Day was a major American holiday. (No pun intended, but one hole in the story has to do with why the notoriously harsh guards were in a giving mood that day, but the account is on the U.S. Naval Institute blog if you want to read more.)

So, there's your history lesson--and an explanation of why National Doughnut Day is actually a celebration of a charitable effort that started 100 years ago this year. Enjoy your sweet free treat, guilt-free.