Southwest Airlines cuts 40% off select roundtrip airfares via coupon code "BIGDEAL40" as part of its Big Winter Sale. (The discount can apply to flights for up to eight people.) That's among the best sales on roundtrip airfares we've seen from Southwest Airlines this year. (We saw 50% off in August.) Book by October 11 for travel between November 28 and December 12 or January 7 and February 14. Additional taxes and fees apply. Several routes and blackout dates are excluded.