Weekends and public holidays matter for deadlines and service levels:
=NETWORKDAYS(start_date, end_date, [holidays]) counts the working days between two dates, including both ends, skipping Saturdays and Sundays.=WORKDAY(start_date, days, [holidays]) returns the date that many working days after the start (the start itself doesn't count).
The optional holidays argument is a range of dates to skip as well. Lock it with $ when you fill the formula.
For other weekends, such as Friday and Saturday, the .INTL versions take a weekend code: NETWORKDAYS.INTL and WORKDAY.INTL.