For those of you who may find yourselves in Rome, the 7th International Pilgrimage to Rome of the People of Summorum Pontificum is happening Friday, 26 October through Sunday, 28 October. Can’t make it to Rome? Well, we can at least review Summorum pontificum.
What, you ask, is Summorum Pontificum? It is the document, promulgated by Pope Benedict XVI on 7 July 2007, found at the Vatican website here and reproduced in its entirety for your convenience as follows:
POPE BENEDICT XVI
GIVEN MOTU PROPRIO
ON THE USE OF THE ROMAN LITURGY
PRIOR TO THE REFORM OF 1970
The Supreme Pontiffs have to this day shown constant concern that the Church of Christ should offer worthy worship to the Divine Majesty, “for the praise and glory of his name” and “the good of all his holy Church.”
As from time immemorial, so too in the future, it is necessary to maintain the principle that “each particular Church must be in accord with the universal Church not only regarding the doctrine of the faith and sacramental signs, but also as to the usages universally received from apostolic and unbroken tradition. These are to be observed not only so that errors may be avoided, but also that the faith may be handed on in its integrity, since the Church’s rule of prayer (lex orandi) corresponds to her rule of faith (lex credendi).” 
Eminent among the Popes who showed such proper concern was Saint Gregory the Great, who sought to hand on to the new peoples of Europe both the Catholic faith and the treasures of worship and culture amassed by the Romans in preceding centuries. He ordered that the form of the sacred liturgy, both of the sacrifice of the Mass and the Divine Office, as celebrated in Rome, should be defined and preserved. He greatly encouraged those monks and nuns who, following the Rule of Saint Benedict, everywhere proclaimed the Gospel and illustrated by their lives the salutary provision of the Rule that “nothing is to be preferred to the work of God.” In this way the sacred liturgy, celebrated according to the Roman usage, enriched the faith and piety, as well as the culture, of numerous peoples. It is well known that in every century of the Christian era the Church’s Latin liturgy in its various forms has inspired countless saints in their spiritual life, confirmed many peoples in the virtue of religion and enriched their devotion.
In the course of the centuries, many other Roman Pontiffs took particular care that the sacred liturgy should accomplish this task more effectively. Outstanding among them was Saint Pius V, who in response to the desire expressed by the Council of Trent, renewed with great pastoral zeal the Church’s entire worship, saw to the publication of liturgical books corrected and “restored in accordance with the norm of the Fathers,” and provided them for the use of the Latin Church.
Among the liturgical books of the Roman rite, a particular place belongs to the Roman Missal, which developed in the city of Rome and over the centuries gradually took on forms very similar to the form which it had in more recent generations.
“It was towards this same goal that succeeding Roman Pontiffs directed their energies during the subsequent centuries in order to ensure that the rites and liturgical books were brought up to date and, when necessary, clarified. From the beginning of this century they undertook a more general reform.”  Such was the case with our predecessors Clement VIII, Urban VIII, Saint Pius X , Benedict XV, Pius XII and Blessed John XXIII.
In more recent times, the Second Vatican Council expressed the desire that the respect and reverence due to divine worship should be renewed and adapted to the needs of our time. In response to this desire, our predecessor Pope Paul VI in 1970 approved for the Latin Church revised and in part renewed liturgical books; translated into various languages throughout the world, these were willingly received by the bishops as well as by priests and the lay faithful. Pope John Paul II approved the third typical edition of the Roman Missal. In this way the Popes sought to ensure that “this liturgical edifice, so to speak ... reappears in new splendour in its dignity and harmony.” 
In some regions, however, not a few of the faithful continued to be attached with such love and affection to the earlier liturgical forms which had deeply shaped their culture and spirit, that in 1984 Pope John Paul II, concerned for their pastoral care, through the special Indult Quattuor Abhinc Annos issued by the Congregation for Divine Worship, granted the faculty of using the Roman Missal published in 1962 by Blessed John XXIII. Again in 1988, John Paul II, with the Motu Proprio Ecclesia Dei, exhorted bishops to make broad and generous use of this faculty on behalf of all the faithful who sought it.
Given the continued requests of these members of the faithful, long deliberated upon by our predecessor John Paul II, and having listened to the views expressed by the Cardinals present at the Consistory of 23 March 2006, upon mature consideration, having invoked the Holy Spirit and with trust in God’s help, by this Apostolic Letter we decree the following:
Art 1. The Roman Missal promulgated by Pope Paul VI is the ordinary expression of the lex orandi (rule of prayer) of the Catholic Church of the Latin rite. The Roman Missal promulgated by Saint Pius V and revised by Blessed John XXIII is nonetheless to be considered an extraordinary expression of the same lex orandi of the Church and duly honoured for its venerable and ancient usage. These two expressions of the Church’s lex orandi will in no way lead to a division in the Church’s lex credendi (rule of faith); for they are two usages of the one Roman rite.
It is therefore permitted to celebrate the Sacrifice of the Mass following the typical edition of the Roman Missal, which was promulgated by Blessed John XXIII in 1962 and never abrogated, as an extraordinary form of the Church’s Liturgy. The conditions for the use of this Missal laid down by the previous documents Quattuor Abhinc Annos and Ecclesia Dei are now replaced as follows:
Art. 2. In Masses celebrated without a congregation, any Catholic priest of the Latin rite, whether secular or regular, may use either the Roman Missal published in 1962 by Blessed Pope John XXIII or the Roman Missal promulgated in 1970 by Pope Paul VI, and may do so on any day, with the exception of the Easter Triduum. For such a celebration with either Missal, the priest needs no permission from the Apostolic See or from his own Ordinary.
Art. 3. If communities of Institutes of Consecrated Life and Societies of Apostolic Life, whether of pontifical or diocesan right, wish to celebrate the conventual or community Mass in their own oratories according to the 1962 edition of the Roman Missal, they are permitted to do so. If an individual community or an entire Institute or Society wishes to have such celebrations frequently, habitually or permanently, the matter is to be decided by the Major Superiors according to the norm of law and their particular laws and statutes.
Art. 4. The celebrations of Holy Mass mentioned above in Art. 2 may be attended also by members of the lay faithful who spontaneously request to do so, with respect for the requirements of law.
Art. 5, §1 In parishes where a group of the faithful attached to the previous liturgical tradition stably exists, the parish priest should willingly accede to their requests to celebrate Holy Mass according to the rite of the 1962 Roman Missal. He should ensure that the good of these members of the faithful is harmonized with the ordinary pastoral care of the parish, under the governance of the bishop in accordance with Canon 392, avoiding discord and favouring the unity of the whole Church.
§2 Celebration according to the Missal of Blessed John XXIII can take place on weekdays; on Sundays and feast days, however, such a celebration may also take place.
§3 For those faithful or priests who request it, the pastor should allow celebrations in this extraordinary form also in special circumstances such as marriages, funerals or occasional celebrations, e.g. pilgrimages.
§4 Priests using the Missal of Blessed John XXIII must be qualified (idonei) and not prevented by law.
§5 In churches other than parish or conventual churches, it is for the rector of the church to grant the above permission.
Art. 6. In Masses with a congregation celebrated according to the Missal of Blessed John XXIII, the readings may be proclaimed also in the vernacular, using editions approved by the Apostolic See.
Art. 7. If a group of the lay faithful, as mentioned in Art. 5, §1, has not been granted its requests by the parish priest, it should inform the diocesan bishop. The bishop is earnestly requested to satisfy their desire. If he does not wish to provide for such celebration, the matter should be referred to the Pontifical Commission Ecclesia Dei.
Art. 8. A bishop who wishes to provide for such requests of the lay faithful, but is prevented by various reasons from doing so, can refer the matter to the Pontifical Commission Ecclesia Dei, which will offer him counsel and assistance.
Art. 9, §1 The parish priest, after careful consideration, can also grant permission to use the older ritual in the administration of the sacraments of Baptism, Marriage, Penance and Anointing of the Sick, if advantageous for the good of souls.
§2 Ordinaries are granted the faculty of celebrating the sacrament of Confirmation using the old Roman Pontifical, if advantageous for the good of souls.
§3 Ordained clerics may also use the Roman Breviary promulgated in 1962 by Blessed John XXIII.
Art. 10. The local Ordinary, should he judge it opportune, may erect a personal parish in accordance with the norm of Canon 518 for celebrations according to the older form of the Roman rite, or appoint a rector or chaplain, with respect for the requirements of law.
Art. 11. The Pontifical Commission Ecclesia Dei, established in 1988 by Pope John Paul II , continues to exercise its function. The Commission is to have the form, duties and regulations that the Roman Pontiff will choose to assign to it.
Art. 12. The same Commission, in addition to the faculties which it presently enjoys, will exercise the authority of the Holy See in ensuring the observance and application of these norms.
We order that all that we have decreed in this Apostolic Letter given Motu Proprio take effect and be observed from the fourteenth day of September, the Feast of the Exaltation of the Holy Cross, in the present year, all things to the contrary notwithstanding.
Given in Rome, at Saint Peter’s, on the seventh day of July in the year of the Lord 2007, the third of our Pontificate.
BENEDICTUS PP. XVI
This document was accompanied by a “Letter of Instruction” to Bishops. It was published the same day, is found at the Vatican website here, along with further clarification from the Pontifical Commission Ecclesia Dei (promulgated 30 April 2011) here. A couple of tidbits from the 2011 letter:
6. The Roman Missal promulgated by Pope Paul VI (the Novus ordo missae – TC) and the last edition prepared under Pope John XXIII (the “1962 Missal” – TC), are two forms of the Roman Liturgy, defined respectively as ordinaria and extraordinaria (“Ordinary Form” or OF, “Extraordinary Form” or EF – TC): they are two usages of the one Roman Rite, one alongside the other. Both are the expression of the same lex orandi of the Church. On account of its venerable and ancient use, the forma extraordinaria is to be maintained with appropriate honor…
The coetus fidelium (cf. Motu Proprio Summorum Pontificum, art. 5 § 1)
15. A coetus fidelium (“group of the faithful”) can be said to be stabiliter existens (“existing in a stable manner”), according to the sense of art. 5 § 1 of the Motu Proprio Summorum Pontificum, when it is constituted by some people (Note well: no number is given – TC) of an individual parish who, even after the publication of the Motu Proprio, come together by reason of their veneration for the Liturgy in the Usus Antiquior, and who ask that it might be celebrated in the parish church or in an oratory or chapel; such a coetus (“group”) can also be composed of persons coming from different parishes or dioceses, who gather together in a specific parish church or in an oratory or chapel for this purpose.
16. In the case of a priest who presents himself occasionally in a parish church or an oratory with some faithful, and wishes to celebrate in the forma extraordinaria, as foreseen by articles 2 and 4 of the Motu Proprio Summorum Pontificum, the pastor or rector of the church, or the priest responsible, is to permit such a celebration, while respecting the schedule of liturgical celebrations in that same church…
There’s more, of course. There’s clarification regarding special qualifications for priests (none, although obviously a priest unfamiliar with it must learn to say it, cf. para. 20), liturgical and ecclesiastical disciplines (e.g., can “new saints” be inserted into the Missal? Yes. Cf. para. 24-28), use for confirmation (yes, para.29), use during the Sacred Triduum (yes – cf. para. 33, including specifically “…the parish priest or Ordinary, in agreement with the qualified priest, should find some arrangement favourable to the good of souls, not excluding the possibility of a repetition of the celebration of the Sacred Triduum in the same church.”)
The website for the Pontifical Commission “Ecclesia Dei” is here. There’s some interesting stuff there, such as this Letter regarding celebration of marriages by the Society of St. Pius X (yes, “insofar as possible…”).
Curate, ut valeatis.