Southwest Airlines just launched another incredible flash sale , with one-way fares as low as $69.The new spring deals extend to all Southwest destinations within the United States. That's nearly 100 cities, and the deals are insane (though all prices are for one-way tickets). You can travel from Phoenix to Denver for $87; Boston to Chicago for $89; or pull a Master of None and fly from New York to Nashville for $121.So, what's the catch? You have to purchase your ticket by 11:59 p.m. EST on Thursday, March 3. And the deals only apply to trips taken between March 8 through June 15 of this year, and require that you fly on certain days of the week. Discounted flights to and from San Juan, Puerto Rico (and other international destinations) are valid from March 29 through May 25.But we haven't even told you the best part. If you've already purchased a more expensive plane ticket on the carrier for travel during these dates, you can change them here to take advantage of the lower fares. Spring break trip , anyone?