Absolutely any holiday can be twisted into an excuse for a sale. This weekend is no exception. There are Labor Day sales everywhere you look. Frontier Airlines is in on it, offering a damn good deal if you're looking for a flight inside the United States.
The budget airline is offering 90% off flights taken by October 4 if you use the code "NOW90," and 50% off flights taken between October 6 and February 13, 2019 with the code "LATER50." You'll have to purchase before midnight on September 3 if you're going to take advantage of the sale.
The low-cost carrier offers remarkably cheap tickets even without sale prices. Right now, you can find one-way flights for as little as $20 before you take a percentage off the fare, and there are round-trip tickets for $38.40. (Though, the percentage off is on the airfare and does not apply to taxes and carrier fees.)
The cheapest flights are over shorter distances, and the 90% off isn't making a huge dent in your total, as you can see in the example below. However, that discount could take a solid chunk out of a more costly trip.