White Castle Free Cheese Slider September 18th The links in the post below may be affiliate links. Read the Full DisclosureSeptember 17, 2019 by Kim Julian Leave a Comment White Castle is offering a coupon for one Free Cheese Slider – With Any Purchase on September 18th. This is for use in-castle only so you’ll have to go in to get your order. For more bargains check out the Restaurant Deals Page! Follow along with all of the Coupons, Freebies and Deals by Subscribing to STL Mommy’s once daily e-mail or following along on Facebook!