Frontier Airlines is running a one-day promotion, with one-way flight prices beginning at $19.These rock-bottom fares come with plenty of terms and conditions, however.To take advantage of the fares, you must travel on a Tuesday, Wednesday or Saturday from Sept. 29 through Dec. 14, excluding Thanksgiving week. You must also be a paid member of Frontier’s Discount Den membership program. Surcharges will apply for carry-on bags and seat assignments.Sign up for our daily newsletterStill, if you can find times and destinations that work with your schedule, and can navigate the surcharges, these fares represent a great opportunity to take a cheap fall holiday.For more details on flights and terms and conditions, read below.For more TPG news delivered each morning to your inbox, sign up for our daily newsletter.Deal basicsAirline: Frontier.How to book: Directly with the airline.Routes: On selected routes including flights from Chicago, Denver, Fort Lauderdale, Las Vegas, Tampa, Orlando, Phoenix, Philadelphia, San Francisco and St. Louis.Travel dates: Sept. 29 through Dec. 14 on Tuesdays, Wednesdays or Saturdays, excluding Thanksgiving week.Book by: Sept. 8 at 11:59 p.m. EDT.Note: To book most of these $19 fares, you must be a member of Frontier’s Discount Den travel club (which comes with a $59.99 annual fee).Related: How to change or cancel a Frontier Airlines flightSample flightsFRONTIER AIRLINESFrontier only offers the $19 tickets on certain routes and dates. To find a summary of available routes on Frontier’s website, search sample routes from your city of choice, or click on Frontier’s “Deals” menu tab, then enter $19 in the “Budget” box. If your city pops up on the menu, click on the “Book Now” button to grab one of those super-discounted tickets.The promotion indicates you must be a member of the Discount Den membership program to use the $19 fares. However, TPG found multiple routes offering $19 fares for the general public.Before booking, be aware of potentially sizable surcharges for seat assignments and baggage — even for carry-on bags. You’ll have to click through the booking process before surcharges are revealed for the flight. Prices can be $60 for a single carry-on and $50 for seat assignments.Related: Frontier’s baggage fees and how to avoid paying themPhoenix and Denver to Las VegasFRONTIER AIRLINESLooking for a super-cheap Vegas getaway? The above screenshot shows multiple October dates when Frontier offers $19 fares to Las Vegas’ Harry Reid International Airport (LAS) from Phoenix Sky Harbor International Airport (PHX).In this case, the fare is available for both the general public and Discount Den members, so there’s no need to enroll in the program. Return flights are available for the following Tuesday at multiple times during the day, so you can actually use this fare for a one-week Vegas getaway.FRONTIER AIRLINESSimilarly, you can find mid-October $19 flights between Denver International Airport (DEN) and Las Vegas, with no requirement to join the Discount Den club. The $19 flights tend to be in the early morning or later at night, but if you act quickly, some attractive flight times are still available for select routes.Chicago to Los AngelesFRONTIER AIRLINESOne of the best values in Frontier’s sale is the $19 flight between Chicago’s Midway International Airport (MDW) and Ontario International Airport (ONT) outside of Los Angeles. The $19 fare is only available for Discount Den members, but the $29 public fare is still a great deal to fly nonstop across the country. Even though these are not the primary airports for Chicago and Los Angeles, the extra commute may be worth your time to save possibly hundreds of dollars on a flight.The available dates on this route for the $19 fares are more limited than in other cities, with just one date showing up in early October. But if you can find a couple of dates and times that work for you, this is an inexpensive chance to escape the Chicago fall chill for some Los Angeles sunshine.Maximize your purchaseIf you book this deal, don’t forget to use a credit card that earns bonus points on airfare purchases, such as The Platinum Card® from American Express (5 points per dollar on airfare booked directly with the airline or through American Express Travel, on up to $500,000 on these purchases per calendar year). Or try the Citi Premier® Card or Chase Sapphire Reserve (both 3 points per dollar on airfare). You could also use the Chase Sapphire Preferred Card (2 points per dollar on travel). For more ways to get the most out of your airfare purchases, see this guide.Bottom lineYou can’t beat Frontier’s $19 fare sale for cheap tickets. This one-day special is worth your time to seek out some bargain fall getaways.But keep in mind these fares come with plenty of terms and conditions. You must be a paid member of Frontier’s Discount Den membership program, you must fly on a Tuesday, Wednesday or Saturday and you’ll have to pay (sometimes sizable) surcharges for seat assignments and baggage.Even if you’re not a member of Frontier’s membership program, the $29-$39 nonmember one-way fares represent good deals — and some $19 fares are available for the general public as well.So if you can find Frontier flights that fit your schedule and can manage without luggage, you should snap up a few of these flights to take some ultra-cheap fall trips.