jubilee

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A specially celebrated anniversary, especially a 50th anniversary.
• noun The celebration of such an anniversary.
• noun A season or occasion of joyful celebration.
• noun Jubilation; rejoicing.
• noun Bible In the Hebrew Scriptures, a year of rest to be observed by the Israelites every 50th year, during which slaves were to be set free, alienated property restored to the former owners, and the lands left untilled.
• noun Roman Catholic Church A year during which plenary indulgence may be obtained by the performance of certain pious acts.

from The Century Dictionary.

• noun Among the ancient Jews, according to the law in Lev. xxv., a semi-centennial epoch of general restoration and emancipation, when liberty was to be proclaimed throughout the land with the blowing of trumpets.
• noun In the Roman Catholic Church, a year in which remission from the penal consequences of sin is granted by the church to those who repent and perform certain acts.
• noun Now, in general, the completion of the fiftieth year of any continuous course of existence or activity, or a celebration of the completion of fifty years, whether on the anniversary day or in a succession of festivities or observances: as, the jubilee of a town or of a, pastorate; the jubilee of Queen Victoria.
• noun Hence Any exceptional season or course of rejoicing or festivity; a special occasion or manifestation of joyousness.
• noun The fiftieth year; the year following any period of forty-nine (or sometimes fifty) years.
• noun A period of fifty years; a half-century.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Jewish Hist.) Every fiftieth year, being the year following the completion of each seventh sabbath of years, at which time all the slaves of Hebrew blood were liberated, and all lands which had been alienated during the whole period reverted to their former owners.
• noun The joyful commemoration held on the fiftieth anniversary of any event
• noun (R. C. Ch.) A church solemnity or ceremony celebrated at Rome, at stated intervals, originally of one hundred years, but latterly of twenty-five; a plenary and extraordinary indulgence granted by the sovereign pontiff to the universal church. One invariable condition of granting this indulgence is the confession of sins and receiving of the eucharist.
• noun A season of general joy.
• noun rare A state of joy or exultation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a special anniversary (or the celebration of it)