I received an e-mail this morning tied to my Potbelly's app account.
"Happy Thursday Potbelly Nation! Buy any Originals, Skinnys or FLATS sandwich and get one free today for a friend (or to keep for yourself)."
"To redeem this Perk, be sure you're logged into your Potbelly Perks account on the mobile app or online before ordering or scanning in-shop. Simply add 2 Originals, Skinnys or FLATS sandwiches plus toppings to your order and the second sandwich of equal or lesser value will be deducted from your total at checkout. This offer is good for today, Thursday, December 28, 2017 only."