If you select a 5-day working week this equals that Saturday and Sunday are days off. Hence, vacation booked over a weekend is not counted.
If your company needs a different work week for all employees or selected employee groups you have the option to select a 7-day work week.
When selecting a 7-day work week Saturday and Sunday become working days and if vacation is booked over these days the vacation balance will be credited.
When selecting 7-day - you have the option to select if for example Friday, Saturday or Sunday should be days off.
Click: Setup Admin > Leave & absence: