Slider fans, it's time to celebrate: May 15 is National Slider Day. To celebrate, White Castle is giving away a free cheese slider to anyone who visits Sunday. The freebie is a thank you to all devoted fans of the chain, which has been slinging its miniature burgers for a century. And while White Castle doesn't even have all that many locations west of the Mississippi or south of Tennessee, what it does have is some interesting history, a huge cult following, and a stoner movie — "Harold & Kumar Go To White Castle" — that put it directly into the public consciousness. Here are some things you might not have known about this buzzy fast-food fixture.