Qu'est-ce que tu vas faire ce week-end ? - What are you going to do this weekend?
I suspect that part of the reason that the term week-end had to be imported into French was that the concept did not even exist for a long time: in France, the weekend is not always as free and clear for students and many employees as it is in other countries. French stores and banks are usually open on Saturday and, until 2008, so were schools (a change which has been debated ever since and might be reversed - read more). When there was just one free day (Sunday) for students and many workers, the idea of the weekend—which really means "weekend off"—was far less important. (On the other hand, the French often turn one-day holidays into four-day weekends, when they font le pont)
Note: Though you will hear week-end in France (and possibly other French-speaking countries), it is not a French word: it's franglais, which automatically makes it informal. Use with caution, if at all.
Official French equivalent: la fin de semaine
Click the little graphic below to hear the franglais term week-end pronounced in French.