All of the super-cheap trips must take place between November 29, 2017 and February 13, 2018. There's a blackout period over the holidays in which the deal isn't applicable, ending January 3. All you had to do was enter the incredibly apt promo code JUST4U and book your travel for half the price of an evening at TGI Friday's. Spirit, which like many other budget airlines, charges for the most basic flight amenities among other fees, so always check the fine print before you get too excited about flying. There sale page also lists several routes that aren't included in the discount rates, which you can take a look at here.
And, yes, you had only until midnight on Thursday, November 16, (or before they sold out) to book the cheap flights. But don't worry, as airlines will likely unleash a few more sales to help you get the hell out of dodge for cheap before the year is up.