Question: Why does the calendar of saints' feast days vary from one traditional Catholic church to another? If they use the same Mass, why not the same calendar? Wasn't the calendar made permanent by Pope St. Pius V? DMR-Boca Raton
Answer: Under normal conditions, Catholics belonging to the Roman rite would all base their calendar of feast days on the calendar employed in Rome. However, every calendar based on the Roman model will have its own particular variations. Each diocese will place special emphasis on its patron saint(s) and on the anniversary day of the dedication of the cathedral. For example, on March 17th in New York it will be Saint Patrick's Day, even though most of the Catholic Church is observing some Ferial day of Lent -- the cathedral is named in honor of St. Patrick. A similar variation will be found at the parish level, with St. Swithin's church celebrating their dedication date and the feast of St. Swithin, even though his feast is ignored in most other churches.
Patronal saints may have proper feast days in the calendars of cities, states, nations, provinces, and episcopal conferences. For example, in these United States and Canada, we observe the feast of the North American Martyrs on September 26th. The French and the Canadians honor St. Joan of Arc on the 30th of May. Religious orders have their own proper calendars, and these too may vary with location.
Religious orders with a large number of canonized saints may diverge widely from the Roman calendar. Some orders (and some cities) don't follow the Roman rite, let alone the Roman calendar. The Dominicans and the Carthusians are such orders; Milan and Lyons are such cities. Generally, they follow the Roman calendar for Sundays and the seasons of the year, but not for most of the saints' days.
An appreciable number of Catholics belong to Eastern rites, with their own proper Masses, missals, and calendars. Again there is some similarity in the seasonal calendar, but very little in the calendar of the saints.
Pope Saint Pius V did fix the rite of Mass for the Roman rite "in perpetuity," forbidding essential changes in the text of the Mass. He did not do the same for the calendar. In fact, in his own time, Pope St. Pius added the feast of Our Lady of the Rosary to the calendar in celebration of the victory of the Catholic navies over the forces of heathen Islam. Christian piety demands a certain dynamic in the calendar in order to accommodate the gradually shifting emphasis in Catholic devotion. For example, devotions to the Immaculate Heart of Mary and the Sacred Heart of Jesus have evolved over the past few hundred years, and have been honored with liturgical feasts. Most likely, similar additions to the calendar will take place in the future, accompanied by the displacement of feasts considered to be less important.
This answer started out with the phrase "Under normal conditions." The near apostasy in the Church is certainly not its "normal condition." In order to preserve the Mass and the teachings of the Faith, several ad hoc organizations formed to deal with what Pope Paul VI called "the autodemolition of the Church." Each of these organizations made its own decisions as to when the calendar and rubrics of the Mass departed from their normal organic development and began to decay into the Novus Ordo Missae or New Order of Mass.
In particular, traditional churches have reacted negatively to the replacement of the feasts of the saints and the Blessed Virgin with Masses of the season (e.g. Tuesday of the 37th week in Ordinary Time). Again, the reaction has not been uniform but will probably remain until order is restored to the Church.