In a bit of confusion, they're referring to the deal as the "$6 All Classic Footlong Event," (they're also calling it "Sub-mageddon") but the "classic" actually refers to all regular-priced Subway sandwiches without upgrades (i.e. extras or Deluxe) rather than a specific tier of their sandwich menu.
Footlongs that normally cost less than $6 will remain the same price during the promotion.
The $6 Footlong deal is good at participating Subway locations throughout February (after the 4th). If you live in Alaska or Hawaii, it will cost more (but you're likely used to it by now).
