Let me state my bias first. I am not a fan of the New Calendar and not because it is of papal origins or secularizing influence or anything else. It breaks things (see here for more on that) and until we move fully back to the Old or fully into something New, it won't be rectified. This bit didn't make it into the below article by Fr. Lawrence Farley, but a lot else did. Enjoy.
(OCA) - Christmas Day and the post-Christmas season usually bring with them a number of things not overwhelming helpful—Boxing Day stampedes, post-Christmas let-down, unwelcome news when stepping on the bathroom scale, and polemical digs about those benighted people using the “papal calendar” instead of “the Church’s Traditional Calendar”—i.e. the Julian calendar. It can be rather confusing to those outside of Orthodoxy, especially when they have been told that Orthodox Christians celebrate Christmas on January 7, thirteen days later than Christians of the West. When I tell them that many Orthodox celebrate Christmas with other Christians on December 25 and that even those Orthodox who use the Julian calendar also celebrate Christmas on December 25 but just don’t get around to that date until January 7, their eyes tend to glaze over. I suspect they conclude that we are all a bit crazy, and the mysteries of the Orthodox calendar partake of the same mind-numbing incomprehensibility as our doctrine of the Trinity, so that for us three=one, and December 25=January 7. In fairness to them, it can be a bit confusing.Complete article here.
In sorting the thing out, it is important not to let triumphalist rhetoric detach us from the sober facts of history. For example, contrary to what some fervent advocates of the Julian calendar sometimes say, the Council of Nicea did not in fact mandate the use of the Old Calendar, or in fact any particular civil calendar. Though it does not show up in the 20 extant canons of that Council, most historians nonetheless assert that the Council did however mandate something regarding the computation of Pascha so that all the Church could fast and feast together. The history of the Council is complex and those wanting to learn more about its intricacies may read about them on the OCA web site.
Briefly, the Church eventually decided that Pascha would be held on the Sunday after the first full moon after the spring equinox. That of course left the astronomical heavy lifting of determining exactly when the spring equinox fell to others. Such technicalities and the question of which civil calendar the Church used were not broached by the Council Fathers. The Church calendar was a grid, something to be placed over the civil calendar of the day to tell Christians when to celebrate certain feasts. It would say, for example, that Christmas must be celebrated on December 25, that Theophany must be celebrated on January 6, and that Transfiguration must be celebrated on August 6. The question about exactly when December 25, January 6, and August 6 fell were matters for the astronomers producing civil calendars, not for non-astronomical bishops leading their flocks in worship.
In the centuries following, it was apparent to all that the civil calendar upon which the Church’s calendar was based was astronomically out of whack and becoming more out of whack with the passing of time and needed to be corrected and made more astronomically accurate. The job, of course, was one for the universities, and their help was solicited by the Roman Catholic Church in the 16th century. A suggestion for correction was made by the University of Salamanca in 1515, which was not acted upon. In 1577 certain mathematicians were asked to weigh in. Others also weighed in, including one Christopher Clavius, who argued the technicalities in a door-stopper of a book stretching to 800 pages. The Pope of the day, Gregory XIII, thought this was the way to go, and mandated the new corrected calendar and system for use in the Roman Church in 1582...