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 need a different work week for all employees or selected employee groups you have the option to select 7-day work week.
When selecting 7-day work week saturday and sunday becomes 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 exampel friday, saturday or sunday should be days off.
Click: Setup Admin > Leave & absence: