When is Yovel? Is it in the 49th year of the shmita cycle, or the 50th? And if it is in the 50th, when is the next shmita, in the 56th or 57th year? What is the practice in modern day Israel?
Yovel is not practiced nowadays, because it applies only when most of the Jewish People live in the Land of Israel, each in their designated tribal territories (Rambam, Hil. Shemittah Veyovel 10:8).
When it is in effect, there is a halachic dispute as to whether it breaks up the cycles of shemittah years (i.e., you have 7 shemittah cycles totaling 49 years, then a year of yovel, and then the next 49-year cycle) or not (i.e., that the yovel is also the first year of the next 49-year cycle, thus being the 50th year inclusive from the previous one). Rambam rules according to the latter opinion; this would mean that the next shemittah year would be the 57th. Again, though, nowadays shemittah years just follow each other every 7th year without a break.
How the shemittah year is calculated is itself the subject of a dispute among the Rishonim. Present-day practice again follows the Rambam, who says that the year of the destruction of the second Beis Hamikdash was 3829 since Creation, and that this was a shemittah year; accordingly, the last one was 5768 and the next one is 5775.