The Church’s calendar depends considerably upon the calendar in ordinary use at the present time, which is called the Gregorian Calendar, from Pope Gregory XIII, who brought it to its present form. The arrangement of the year devised by this great Pontiff is so admirable that it may be well to give a brief explanation of it and of lead our readers a little way into the paths of astronomical science.
The sun is the center of our planetary system, and the earth travels around it in what we call a year, turning at the same time on its own axis, each complete turn constituting what we call a day. But the journey of the earth around the sun does not happen to be completed in an exact number of days. If it leaves a certain point in its track or orbit on January first, for instance, it is not at that precise point at the same hour on the following January first. Hence arises the necessity of having “leap-years,” so that our calendar may be brought into close agreement with the real year of the earth traveling around the sun. Otherwise, the difference between the real year and the year of 365 days would gradually cause the seasons to shift — until Christmas Day, for example, would occur in midsummer in our part of the world.
Leap-years were invented by Julius Caesar, who estimated the length of the solar year as 363 days and six hours; and to provide for these extra six hours he inserted an additional day into each fourth year. But his estimate of the year’s length was not precise; the year is really less than the above figures by eleven minutes and fourteen seconds. The result was that there was an error of a full day in about 134 years.
Pope Gregory’s Reform. In 1582 Gregory XIII brought the calendar to its present form by the simple plan of dropping ten days, from October 5 to 14 inclusively, for the error had increased to that extent since Caesar’s time. The Pontiff provided for the future by ordering that the leap-year should not be observed in 1700, 1800 and 1900, but should be retained in the year 2000 and in every century thereafter that is divisible by 400. This ingenious method gives such a close approach to exactness that only after thirty-five centuries will there be an error of one day — and that is too remote to be of much concern to you or me. ,
But this was a Roman decree — and therefore it was bitterly opposed by the Protestant countries of Europe. Nearly all of them, however, adopted it in the year 1700. England, with true British obstinacy, held on to the old style until 1752, when she was eleven days “behind schedule.” Russia still adheres to the Julian calendar, and is now thirteen days behind the rest of the world.
Such is a brief history of the Gregorian calendar. As the years roll on — those secular years which the wisdom of one of the Church’s rulers has brought into close accord with the unvarying
movements of God’s universe — day by day the Church keeps her own calendar. She celebrates the great events in the history of the world’s redemption. She honors the Mother of God and the faithful ones who have served their Master well. She has her seasons of penance and her festivals of joy. And so shall her years be kept while this earth endures — until the day shall come when “ the heavens shall be folded together like a scroll ” and time shall be no more.
Sullivan, John F., The Externals of the Catholic Church: A Handbook of Catholic Usage, New York: P.J. Kenedy & Sons, 1951, p. 168