Why a jubilee? The first paragraph explains it:
3. At times we are called to gaze even more attentively on mercy so that we may become a more effective sign of the Father’s action in our lives. For this reason I have proclaimed an Extraordinary Jubilee of Mercy as a special time for the Church; a time when the witness of believers might grow stronger and more effective.
The Church is called to a greater “effectiveness.” I think the term fruitfulness may also capture something of a desire that the Church would, somehow, have far more impact on believers, seekers, and non-believers alike. Will the year be experienced as a “special time”? Time will tell.
The beginning of the year is described, including the notion that a certain portal will not only be described by its sanctity, but also by mercy:
The Holy Year will open on 8 December 2015, the Solemnity of the Immaculate Conception. This liturgical feast day recalls God’s action from the very beginning of the history of (humankind). After the sin of Adam and Eve, God did not wish to leave humanity alone in the throes of evil. So he turned his gaze to Mary, holy and immaculate in love (cf. Eph 1:4), choosing her to be the Mother of (our) Redeemer. When faced with the gravity of sin, God responds with the fullness of mercy. Mercy will always be greater than any sin, and no one can place limits on the love of God who is ever ready to forgive. I will have the joy of opening the Holy Door on the Solemnity of the Immaculate Conception. On that day, the Holy Door will become a Door of Mercy through which anyone who enters will experience the love of God who consoles, pardons, and instills hope.
As I look upon two vital qualities of this last sentence, I’m reminded of the topics of love and hope so well covered by the pope emeritus.
St Peter’s will not have the only door of mercy:
On the following Sunday, the Third Sunday of Advent, the Holy Door of the Cathedral of Rome – that is, the Basilica of Saint John Lateran – will be opened. In the following weeks, the Holy Doors of the other Papal Basilicas will be opened. On the same Sunday, I will announce that in every local Church, at the cathedral – the mother church of the faithful in any particular area – or, alternatively, at the co-cathedral or another church of special significance, a Door of Mercy will be opened for the duration of the Holy Year. At the discretion of the local ordinary, a similar door may be opened at any Shrine frequented by large groups of pilgrims, since visits to these holy sites are so often grace-filled moments, as people discover a path to conversion. Every Particular Church, therefore, will be directly involved in living out this Holy Year as an extraordinary moment of grace and spiritual renewal. Thus the Jubilee will be celebrated both in Rome and in the Particular Churches as a visible sign of the Church’s universal communion.
Has this occurred in other jubilee years, the designation of doors at cathedrals, shrines, and other sites? Suppose your parish were asked to designate a mercy door. What would that look like? How would it be introduced? Would it be seen as a significant connection to the universal church? If so, how?
The highlighted text is © copyright – Libreria Editrice Vaticana. You can find the document in its entirety on the Vatican website here.
Catholic Sensibility 
At least for the Jubilee in 2000 Cathedrals and maybe some shrines were also allowed to have Holy Doors. I believe there are also a few churches outside of Rome that have been granted the privilege of a permeant holy door that is only opened during Jubilee years.
In 2000, my local Cathedral designated a Holy Door and locked it for several months prior to the start of the Jubilee to allow for more symbolism when it was opened. I must admit I don’t really remember any more details.
In the 2000 Jubilee, each bishop was given the authority, and was encouraged, to designate additional churches in his jurisdiction. IIRC, in the Boston archdiocese, every deanery/vicariate forane had a chuch designated. The oldest parish in my small city was so designated, and, as it was undergoing a major renovation in 1999, had a jubilee window installed over the main entrance, with the phrase “Open Wide The Doors to Christ”.