On National Slider Day, White Castle provides a free Original Slider to each customer who presents a special coupon or uses the promo code "SLIDERDAY" in the White Castle app. This promotion is available at participating locations and aims to celebrate the day with fans of the brand. Additionally, new members who sign up for the Craver Nation in the White Castle app can also receive a free combo meal. 

 White Castle celebrates National Slider Day on May 15th by offering a free Original Slider to customers. No purchase is necessary, and customers can claim this offer by showing a coupon or using a code in the app.