Frontier Airlines is now offering a Cyber Monday deal with 75% off flights. Although I am sure we will see fare sales throughout the week as well since they seem to offer them every other day!
For flights purchased today only, use promo code SAVE75 and you’ll get 75% off the base fare of the flight (which means taxes and other fees are not discounted). This fare excludes Sunday travel and is for November 29 through February 7, 2018. If you are not 100% sure of your plans, you can buy now and cancel within 24 hours (as long as your flight is not within 7 days).
I actually do not mind Frontier flights at all. And really love the extremely discounted fares! While it is a no frills airline and you pay extra for seat assignment and carry on luggage that cannot fit in under the seat, their planes are clean and I find that they are typically on time. In my opinion they are way better then Spirit.