In the Old Covenant, God set aside certain times to be honored as sacred. As recorded by Moses in Exodus 20:8-11, there was to be a weekly "Sabbath" -- which means "cease" or "rest." In Deuteronomy 16:16-17, Moses records God's commands to our spiritual ancestors to keep the yearly Passover, the Feast of Weeks, and the Feast of Tabernacles.

In addition to these weekly and yearly cycles of time, God also ordered periodic year-long sabbaths. These "sabbatical years" were of two types: the regular sabbatical year which was to take place every 7th year, and the special year of Jubilee, which took place after "seven weeks of seven years," or after 49 years -- that is, in every 50th year. All told, then, every 7th, 14th, 21st, 28th, 35th, 42nd, 49th, and 50th years would be sabbath years, with two years in a row -- the 49th year and the 50th year of jubilee -- being such. The word "jubilee" is a Hebrew word which etymologically indicates the ram's horn -- "jobel" (also "shofar") -- that God ordained should announce these special sabbatical years:

Leviticus 25:1-13
And the Lord spoke to Moses in mount Sinai, saying: Speak to the children of Israel, and thou shalt say to them: When you shall have entered into the land which I will give you, observe the rest of the sabbath to the Lord. Six years thou shalt sow thy field and six years thou shalt prune thy vineyard, and shalt gather the fruits thereof: But in the seventh year there shall be a sabbath to the land, of the resting of the Lord: thou shalt not sow thy field, nor prune thy vineyard. What the ground shall bring forth of itself, thou shalt not reap: neither shalt thou gather the grapes of the firstfruits as a vintage: for it is a year of rest to the land: But they shall be unto you for meat, to thee and to thy manservant, to thy maidservant and thy hireling, and to the strangers that sojourn with thee: All things that grow shall be meat to thy beasts and to thy cattle. Thou shalt also number to thee seven weeks of years, that is to say, seven times seven, which together make forty-nine years: And thou shalt sound the trumpet in the seventh month, the tenth day of the month, in the time of the expiation in all your land. And thou shalt sanctify the fiftieth year, and shalt proclaim remission to all the inhabitants of thy land: for it is the year of jubilee. Every man shall return to his possession, and every one shall go back to his former family: Because it is the jubilee and the fiftieth year. You shall not sow, nor reap the things that grow in the field of their own accord, neither shall you gather the firstfruits of the vines, Because of the sanctification of the jubilee: but as they grow you shall presently eat them. In the year of the jubilee all shall return to their possessions.

The lands were let rest ("sabbath"), debts were forgiven, and slaves were freed. These acts preserved the theocratic order of ancient Israel by disallowing one man to accumulate material goods and land at the expense of the community, and by reinforcing the truth that the land belonged not to them, but to God. These years kept things harmonious by making almost impossible the division of Israel into warring classes. But there is something more to these "days of the Lord": they foreshadow the Kingdom, and Isaias prophecied to this:

Isaias 61:1-3
The spirit of the Lord is upon me, because the Lord hath anointed me: he hath sent me to preach to the meek, to heal the contrite of heart, and to preach a release to the captives, and deliverance to them that are shut up. To proclaim the acceptable year of the Lord, and the day of vengeance of our God: to comfort all that mourn: To appoint to the mourners of Sion, and to give them a crown for ashes, the oil of joy for mourning, a garment of praise for the spirit of grief: and they shall be called in it the mighty ones of justice, the planting of the Lord to glorify hint.

And in fulfillment of the Prophet's words, Jesus came:

Luke 4:16-21
And he came to Nazareth, where He was brought up: and He went into the synagogue, according to His custom, on the sabbath day; and He rose up to read. And the book of Isaias the prophet was delivered unto Him. And as He unfolded the book, He found the place where it was written: The Spirit of the Lord is upon me. Wherefore he hath anointed me to preach the gospel to the poor, he hath sent me to heal the contrite of heart, To preach deliverance to the captives, and sight to the blind, to set at liberty them that are bruised, to preach the acceptable year of the Lord, and the day of reward. And when He had folded the book, He restored it to the minister, and sat down. And the eyes of all in the synagogue were fixed on Him. And He began to say to them: This day is fulfilled this scripture in your ears.

Because of the Old Covenant ordinances involving the 7th and 50th years, these time periods also have significance, though spiritualized, in the Christian era. Jubilee anniversaries of religious profession and ordinations and the like were always celebrated, for example, but, as far as we can tell from the Historical record, Pope Boniface VIII is the first Pope to have instituted regular Holy Years of Jubilee with the accompanying indulgences as we know them today. He did so in A.D. 1300, and meant for the Christian Jubilee to be celebrated every 100 years, but a second Jubilee was called for 1350, then in 1390 and 1423. Thereafter, since 1450, Jubilees were set to take place every 25 years, and this is the pattern that's been followed ever since except for the years 1800, when Napoleon ruled, and 1850, when Pope Pius IX was in exile, and no Jubilees were called. Extraordinarily, Popes can call for a Jubilee any time they wish, and sometimes do so at times of great calamity or war, or on special occasions, such as at the convening of Councils or at the anniversaries of great events.

 
What Happens in Jubilee Years

 
As in the Old Covenant, the purpose of Jubilee Years is not only rest and celebration, but forgiveness and the exercise of mercy. What was foreshadowed by the temporal "even-ing out" of debts in the Old Covenant, though, is the greater New Covenant reality that Christ's graces offer us eternal effects. The Sacrament of Confession can be likened to God's granting to us what He commanded to Moses: "remission to all the inhabitants of thy land: for it is the year of jubilee." In these holy years, penance is key (a General Confession in Jubilee Years is recommended by Pope Benedict XIV).

In addition to the special emphasis on the alleviation of the eternal effects of sin through the Sacrament of Confession, the temporal effects of sin are a focus of Jubilee Years, too. A plenary indulgence can be gained, under the usual conditions, by making a pilgrimage to the four primary patriarchal churches in Rome and walking through their Holy Doors (portae sanctae, above), which are symbolic of Christ. This is the standard requirement for the Jubilee indulgence, but the exact requirements (published when the Jubilee is announced) may vary from Jubilee to Jubilee and usually include provisions for visiting local churches, doing charitable works, or fasting, etc.      

The four primary patriarchal churches are those churches that are associated with the Patriarchates of the West, Constantinople, Alexandria, and Antioch. They are:
 

• The Church of St. John Lateran is the cathedral of the Pope and associated with him as Patriarch of the West
• St. Peter's Basilica is associated with the Patriarch of Constantinople
• The Church of St. Paul-without-the-Walls Basilica is associated with the Patriarch of Alexandria
• The Church of St. Mary Major is associated with the Patriarch of Antioch

At each of these patriarchal churches are holy doors which are kept sealed except in Jubilee Years, their unsealing taking place on Christmas Eve and lasting until Christmas Day of the following year (or the following Epiphany). The Holy Doors are opened, and later re-sealed, with great solemnity. The traditional rites for doing so are as follows:

 
Opening the Holy Doors

• The Pope vests in a room of the Apostolic Palace and then, together with the Cardinals, proceeds to the Sistine Chapel. There the sending of the Cardinal Legates for the opening of the other Doors and the adoration of the Blessed Sacrament takes place.

• The procession makes its way to the Holy Door accompanied by the chanting of the Iubilate Deo or the Veni Creator Spiritus.

• The Pope says the prayer Deus qui per Moysem.

• He then takes the hammer, recites the verses Aperite mihi portas iustitiae and strikes the wall covering the Holy Door three times.

• The Pope returns to his seat and says the prayer Actiones nostras.

• The masons continue the work of opening the Door to the chanting of the Psalm Iubilate Deo omnis terra.

• The Pope genuflects at the threshold of the door. - The Pope is the first to pass through the Holy Door as the choir chants the Te Deum laudamus.

• The procession moves towards the altar for the celebration of Vespers.

 
Closing the Holy Doors

• The Pope processes into the Basilica through the Holy Door and presides at Vespers in the Basilica.

• He then sends the Cardinal Legates charged with closing the Doors of the other Basilicas.

• A procession follows, first to the relics and then to the Holy Door, accompanied by the singing of appropriate hymns.

• The relics of the Veronica and the Lance are publicly shown and venerated.

• The Pope is the last to leave by the Holy Door.

• He then blesses the stones and the bricks.

• With the trowel he applies cement to the threshold of the Holy Door and sets in place three bricks and a few gold and silver coins.

• Other bricks are added and then the masons, outside and inside the Basilica, finish the work of closing the Door while the choir chants the hymn Caelestis Urbs Ierusalem.

• The Pope says the prayer Deus qui in omni loco and ascends to the Loggia of the Basilica where he solemnly imparts the Apostolic Blessing.

As of this writing, the most recent Holy Year of Jubilee was called in A.D. 2000. Look for another Holy Year to come in A.D. 2025, or for one called extraordinarily outside of the 25-year cycle. When a Jubilee is called, make a general confession, receive the Eucharist, and either make the pilgrimage to Rome, if possible, or follow the instructions in the decree calling for the Jubilee to discover other ways you can receive indulgences to fulfill the Old Covenant type and most fully receive the graces of "the acceptable year of the Lord."

See some pictures of Pope Pius XII's Jubilee celebrations from Life Magazine, January 9, 1950.