These cities have flights available during both sale windows: Albany, Albuquerque, Atlanta, Austin, Baltimore, Billings, Boston, Bozeman, Buffalo, Burlington, Calgary, Cedar Rapids, Charlotte, Chicago, Cincinnati, Cleveland, Colorado Springs, Columbus, Dallas, Denver, Des Moines, Detroit, Fargo, Fort Lauderdale, Grand Rapids, Green Bay, Hartford, Indianapolis, Kansas City, Las Vegas, Madison, Memphis, Miami, Milwaukee, Minneapolis, Nashville, New Orleans, New York City, Oklahoma City, Orlando, Philadelphia, Phoenix, Providence, Raleigh, San Juan, Syracuse, Tampa, and other cities that may only be listed in one of the two sales.
With any budget airline -- or airline, really -- you should take a look at the extra fees you might face before purchasing your flight. With a discount carrier, you're more likely to see little charges, like fees for choosing your seat on the plane. Of course, there are ways around these, but they aren't for everyone. You can avoid that particular fee, for instance, by having your seat assigned at the airport, but that might not be possible for fliers who are, say, traveling with a child. But if you don't have a kid, you'll probably be fine. Yeah, you might sit by a stinky dude who wants to tell you about all his favorite Pokémon cards. But you're more likely to sit by some boring businessman who is too shy to tell you about his Pokémon cards.
However, if circumstances align for you, this might be an opportunity for a cheap flight during the weeks when people aren't traveling en masse for the holidays.