Mark you calendars, set your alarms, and plan on eating lunch out of the office on Friday, May 15th. White Castle will be celebrating National Slider Day! Right now we can print a coupon valid for Any Two Sliders FREE on National Slider Day! Offer valid for any single Slider on a bun (excludes waffles and toast sandwiches). Not valid with any other coupons, discounts, specials, online orders or offers. One coupon per customer per visit. What a delicious freebie!