Photo: Eliyahu Yosef Parypa (Shutterstock)As a tie-in to their 50th anniversary, Southwest Airlines is offering a limited spring and fall sale with prices starting at $50 each way. While the sale mostly applies to domestic travel, prices for Southwest’s limited selection of international flights are discounted, as well. Here’s what you need to know.How the deal worksThere are some decent fares on offer. Some of the lowest fares include:Atlanta – Nashville for $50Baltimore – Chicago for $50Chicago – Miami for $50Chicago – Nashville for $50Dallas – Chicago for $50Houston – New Orleans for $50Los Angeles – Las Vegas for $50Los Angeles – San Francisco for $50Miami – Atlanta for $50Phoenix – San Diego for $50San Diego – San Francisco for $50These non-refundable, discounted fares can be booked on Southwest’s low-fare booking site up until May 3, and tickets must be purchased 21 days prior to the travel date (in that case, if you want travel mid-May, you’ll want book it now).There are good prices on other domestic routes, too, often in the $60-80 range for one-way flights. Additionally, international routes to Mexico and the Caribbean are discounted by $100 or so, although selection is more limited, based on my own research on their booking site.When to bookThe deal is actually for two seasons of travel and has some days of the week in which the sale doesn’t apply. You can book for spring from May 7 through May, 26; and for fall, from September 7 through November 5 (Hawaii has slightly different dates, August 9 through November 5).G/O Media may get a commissionExcept as otherwise specified, continental U.S. travel is not valid on Fridays and Sundays, international travel is only valid Monday through Thursday (except for Mexico which is Tuesdays through Fridays). It’s a bit confusing, so you might be better off poking around their booking site to see which calendar dates offer the best deals.The CDC continues to urge caution The CDC does not recommend that you travel unless you’re vaccinated, which means a full two weeks after the second dose. Even if you’re vaccinated, you’ll still have to practice social distancing and wear a mask, but quarantining is not required. Be sure to read the CDC’s guidelines before you book a flight.