Southwest Airlines is known for being super cheap. Even with the changes to their rewards program, Southwest has some of the lowest fares in the industry. The trick to getting the cheapest flights is understanding how their fares are calculated and when exactly is the best time to fly on Southwest. There’s a lot that goes into calculating an airline’s cheapest day so we’ll explain it all below. If you happen to be in need of a cheap flight this week, check out these great deals on Tuesday, Wednesday, and Thursday of this week. If you're looking for cheap flight…