While most of your employees may have Saturday and Sunday as weekly off, there would be contract workers or shift employees with different week offs. It is particularly true for call centers, plant operations or retail stores.
How would the system know if an employee is absent or just has a weekly off? You get to define it. You can define employee categories (full time, part-time, contract, temporary, factory etc.) and then assign specific weekly offs for them.
Of course, you can do it at an individual level or as a bulk action – either ways weekly offs can be taken care of easily.