In short, the answer to your question is that the Jubilee year is currently not observed or commemorated. The reasons for this are complex and involve many different opinions on the matter. In the following lines I will attempt to briefly relay the relevant issues.
According to biblical law, the Jubilee is only observed when all twelve tribes of the Jewish nation are living in Israel, as is derived from the verse, “And you shall sanctify the fiftieth year, and proclaim freedom throughout the land for all who live on it,” which implies that the Jubilee is only sanctified when “all who live on it”—meaning, all who are meant to be living there—are in the Land of Israel. Furthermore, the Jubilee is only observed when every tribe is living in the specific part of the land which was it was allotted when the Land of Israel was divided. However, some are of the opinion that the Jubilee is observed as long as there is a partial representation of each tribe, even if most of the tribe is not in Israel.
In the 6th century BCE, the Assyrians conquered the Northern Kingdom of Israel and sent the majority of its population into exile. Those who were deported are historically known as the Ten Lost Tribes.
We are certain that before that point in time the Jubilee was regularly observed. We also know that, with the destruction of the Second Temple and the disbandment of the Sanhedrin (supreme rabbinical court), we ceased to mark the Jubilee year in any form. The periods about which there is a question are the remaining years between the exile of the Ten Tribes and the destruction of the First Temple, and the Second Temple Era.
According to the opinion that partial representation of each tribe is sufficient to fulfill the scriptural requirement, biblically mandated Jubilees were fully observed throughout the periods in question, because there remained a small representation of each tribe in Israel.
However, according to the first opinion mentioned above, with the exile of the Northern Kingdom the required condition for the Jubilee to be sanctified was lost. Thus, the last time there was a biblical requirement to observe the Jubilee was about 150 years before the destruction of the First Temple.
The question remains, however, whether according to this opinion Jubilee years were designated or observed during this time by rabbinic injunction. This is the subject of debate amongst the sages.
As mentioned above, though, today the Jubilee year is neither designated nor observed.
And now for the answer to your question: “When is the next Jubilee year?”
We eagerly await the day when G‑d will bring our entire nation back to our homeland—including the ten “lost” tribes—and we will again resume observing the Jubilee year, as well as so many other mitzvot which we are incapable of performing until that awaited day.
Rabbi Baruch S. Davidson