Quick answer: on a 9/80, you work eight 9-hour days and one 8-hour day over two weeks and get the tenth weekday off: 80 hours in 9 days, about 26 extra days off a year.
How a 9/80 schedule works
Over two weeks you work eight 9-hour days, one 8-hour day and have one weekday off: 80 hours in 9 days instead of 10. Most plans put the day off on alternate Fridays, which gives 26 three-day weekends a year.
Overtime and the workweek
Federal overtime is counted per workweek, not per day, so compressed schedules that stay at 40 hours a week owe no federal overtime. States with daily overtime rules treat long days differently.
Prefer one day off every week? See the 4/10 compressed work week.
Sources: DOL: Overtime pay. Rules checked . Found a problem? Tell us.
Questions
How many days off do you get on a 9/80?
One every two weeks, about 26 a year, less any that land on holidays your employer already closes for.