Calling all McDonald's fans! You can get a Buy 1, Get 1 FREE Filet-O-Fish Sandwich deal when you use Mobile Order & Pay through the McD app! This offer is valid once per week through March 4, 2018. Click this link to download the app if you don't already have it then create a free account. Tap on the “View Deals” section of the app to find this offer, customize your order, and apply the coupon and pay. There's also a few other great deals and many more to come. Enjoy! Valid once per week. McD App download and registration required. Mobile Order & Pay available at participating McDonald’s.