https://pressbooks.pub/travelhistory/chapter/what-is-the-%ef%bd%83%ef%bd%88%ef%bd%85%ef%bd%81%ef%bd%90%ef%bd%85%ef%bd%93%ef%bd%94-day-of-week-to-%ef%bd%82%ef%bd%8f%ef%bd%8f%ef%bd%8b-a-united-flight/
https://pressbooks.pub/travelhistory/chapter/what-is-the-%ef%bd%83%ef%bd%88%ef%bd%85%ef%bd%81%ef%bd%90%ef%bd%85%ef%bd%93%ef%bd%94-day-of-week-to-%ef%bd%82%ef%bd%8f%ef%bd%8f%ef%bd%8b-a-united-flight/