Leviticus 25:8
8 “‘You must count off seven weeks of years, seven times seven years, and the days of the seven weeks of years will amount to 49 years. 9 You must sound loud horn blasts—in the seventh month, on the tenth day of the month, on the Day of Atonement—you must sound the horn in your entire land. 10 So you must consecrate the fiftieth year, and you must proclaim a release in the land for all its inhabitants. That year will be your Jubilee; each one of you must return to his property, and each one of you must return to his clan. 11 That fiftieth year will be your Jubilee; you must not sow the land, harvest its aftergrowth, or pick the grapes of its unpruned vines. 12 Because that year is a Jubilee, it will be holy to you—you may eat its produce from the field.
Jubilees 50:4
2 On Mt. Sinai I told you about the sabbaths of the land and the years of jubilees in the sabbaths of the years, but its year we have not told you until the time when you enter the land which you will possess. 3 The land will observe its sabbaths when they live on it, and they are to know the year of the jubilee. 4 For this reason I have arranged for you the weeks of years and the jubilees — 49 jubilees from the time of Adam until today, and one week and two years. It is still 40 years off (for learning the Lord’s commandments) until the time when he leads them across to the land of Canaan, after they have crossed the Jordan to the west of it.
Notes and References
"... The length of a jubilee is somewhat ambiguous in the Bible: according to Leviticus 25:8, it is a period of 49 years, while according to Leviticus 25:11, 50 years. Te author of Jubilees held by the former interpretation. As a result, each jubilee divides easily into seven subgroups of seven years apiece; these subgroups are conventionally called “weeks,” although “groups of seven” would be a more accurate translation. In asserting that Cain was born “in the third week of the second jubilee,” therefore, Jubilees’ author means that Cain was born sometime after the world’s first jubilee (years 1-49) plus two more “weeks” (years 50-63), but before the start of the fourth week in the year 781 ..."
Kugel, James L. A Walk through Jubilees: Studies in the Book of Jubilees and the World of Its Creation (p. 2) Brill, 2012