Which regional rules of lent fasting or holy days of obligation should a Catholic follow?
The answer to this question must be divided into two parts concerns the rules for the Universal Church and the rules concerning the Particular Church (region, country or diocese) in which one is residing in!
The General Roman Calendar applies to all Catholics of the Roman (Latin) Rite, however each domicile in which a person is living in must follow the ecclesiastical rules and norms which have been approved by Rome for that location (usually known as a particular Liturgical calendar of a country or rigious order, such as Benedictines or Dominicans, etc.).
The General Roman Calendar is the liturgical calendar that indicates the dates of celebrations of saints and mysteries of the Lord (Jesus Christ) in the Roman Rite, wherever this liturgical rite is in use. These celebrations are a fixed annual date; or occur on a particular day of the week (examples are the Baptism of the Lord in January and the Feast of Christ the King in November); or relate to the date of Easter (examples are the celebrations of the Sacred Heart of Jesus and the Immaculate Heart of Mary). National and diocesan liturgical calendars, including that of the diocese of Rome itself as well as the calendars of religious institutes and even of continents, add other saints and mysteries or transfer the celebration of a particular saint or mystery from the date assigned in the General Calendar to another date.
These liturgical calendars also indicate the degree or rank of each celebration: Memorial (which can be merely optional), Feast, or Solemnity. Among other differences, the Gloria is said or sung at the Mass of a Feast but not at that of a Memorial, and the Creed is added on Solemnities.
The last general revision of the General Roman Calendar was in 1969 and was authorized by the motu proprio Mysterii Paschalis of Pope Paul VI. The motu proprio and the decree of promulgation were included in the book Calendarium Romanum, published in the same year by Libreria Editrice Vaticana. This contained also the official document Universal Norms on the Liturgical Year and the Calendar, and the list of celebrations of the General Roman Calendar. Both these documents are also printed (in their present revised form) in the Roman Missal, after the General Instruction of the Roman Missal. The 1969 book also provided a detailed unofficial commentary on that year's revision of the calendar.
The General Calendar is printed, for instance, in the Roman Missal and the Liturgy of the Hours. These are up to date when printed, but additional feasts may be added later. For that reason, if those celebrating the liturgy have not inserted into the books a note about the changes, they must consult the current annual publication, known as the "Ordo", for their country or religious congregation. These annual publications, like those that, disregarding the feasts that are obligatory in the actual church where the liturgy is celebrated, list only celebrations included in the General Calendar, are useful only for the current year, since they omit celebrations impeded because of falling on a Sunday or during periods such as Holy Week and the Octave of Easter.
The feast days of saints celebrated in one country are not necessarily celebrated everywhere. For example, a diocese or a country may celebrate the feast day of a saint of special importance there (e.g., St. Patrick in Ireland, Our Lady of Guadalupe in Mexico, St. Elizabeth Ann Seton in the United States). Likewise, a particular religious institute may celebrate its founder or members of the institute, even if that saint is not listed on the universal calendar or is included in it only with a lower rank. The General Roman Calendar contains only those celebrations that are intended to be observed in the Roman Rite in every country of the world.
This distinction is in application of the decision of the Second Vatican Council: "Lest the feasts of the saints should take precedence over the feasts which commemorate the very mysteries of salvation, many of them should be left to be celebrated by a particular Church or nation or family of religious; only those should be extended to the universal Church which commemorate saints who are truly of universal importance.”
Catholics of the Latin Rite must fast on only two days a year, unless bound by particular rules of a particular country or religious order.
Latin Church sui juris
Contemporary canonical legislation for Catholics of the Latin Church sui juris (who comprise most Catholics) is rooted in the 1966 Apostolic Constitution of Pope Paul VI, Paenitemini, and codified in the 1983 Code of Canon Law (in Canons 1249–1253). According to Paenitemini, the 1983 Code of Canon Law and the Constitution Sacrosanctum Concilium, on Ash Wednesday, Good Friday and where possible, throughout Holy Saturday, both abstinence and fasting are required of Catholics who are not exempted for various reasons. The law of fasting binds all Catholics on from age 18 until age 59. All Fridays of the year, except when a Solemnity falls upon the Friday, are bound by the law of abstinence.
Both Paenitemini and the 1983 Code of Canon Law permitted the Episcopal Conferences to propose adjustments of the laws on fasting and abstinence for their home territories. In some countries, the Bishops' Conferences have obtained from Rome the substitution of pious or charitable acts for abstinence from meat on Fridays except Good Friday. Others abstain from eating meat on Lenten Fridays.
The Personal Ordinariates for former Anglicans reconciled to the Catholic Church follow the discipline of the Latin Church (of which they are a part) including the norms established by the Council of Catholic Bishops in whose territories they are erected and of which their Ordinaries are members. Thus, for example, in England, the norm is abstinence on all Fridays of the year. The Bishop in the United States has emphasized the statements in the USCCB norms "Friday itself remains a special day of penitential observance throughout the year," and "we give first place to abstinence from flesh meat.” The Ember Days have been re-established in the Calendar of the Ordinariates, and as long as a Solemnity does not take precedence, the Ember Fridays in September and Advent are days of obligatory abstinence. Obligatory abstinence on Ember Friday in Lent is included in the universal Lenten discipline, and abstinence on Ember Friday on Whitsuntide is not required, as all days of the Octave of Pentecost are Solemnities.
Autonomous Eastern Catholic Churches
Members of the autonomous Eastern Catholic Churches are obliged to follow the discipline of their own particular church. While some Eastern Catholics try to follow the stricter rules of their Orthodox counterparts, the actual canonical obligations of Eastern Catholics to fast and abstain are usually much more lenient than those of the Orthodox.
Eastern Christians view fasting as one part of repentance and supporting a spiritual change of heart. Eastern Christians observe two major times of fasting, the "Great Fast" before Easter, and "Phillip's Fast" before the Nativity.
The fast period before Christmas is called "Philip's Fast" because it begins after the feast day of St. Philip. Specific practices vary, but on some days during the week meat, dairy products and (in some countries) oil are avoided, while on other days there is no restriction. During approximately the last week before the Nativity, typically meat, dairy, eggs and oil are avoided on all days, meals are moderate in quantity, and no food is taken between meals.