Air travel was a mess over the holidays — in the last 10 days of December, 30,000 flights were canceled. While every airline was affected, one stood out: Southwest, which over the past few decades has transformed how Americans fly, melted down. In the last 10 days of the year, it canceled as many flights as it had done in the previous 10 months. So what went wrong?Guest: Niraj Chokshi, a business reporter for The New York Times.Background reading: Southwest’s crisis shows what can go wrong when a company relied on by millions of people moves too slowly to invest in unglamorous parts of its operation.The airline’s customers incurred thousands in expenses as they scrambled to get home. For more information on today’s episode, visit nytimes.com/thedaily. Transcripts of each episode will be made available by the next workday.