The Church's calendar depends on the calendar in use at the present time, which is known as the Gregorian, from Pope Gregory XIII, by whose decree it was brought to its present form.
As the length of the year depends on the time of the earth's journey around the sun, and as that journey is not completed in exactly 365 days, Julius Cresar made each fourth year a "leap year" by inserting an additional day in February.
The Julian Calendar, however, was inaccurate; the journey of the earth is made in a little less than 365 days, and a constantly increasing error resulted.
In Pope Gregory's time, 1582, it amounted to 10 days; he therefore dropped these from the calendar and ordered that the leap year should be observed in the year 1600, but not in 1700, 1800, and 1900, and that thereafter century years would be leap years only when they are divisible by 400.
This gives a year so nearly exact that there will be an error of one day in 35 centuries.
The calculations were made principally by two astronomers, Lulli and Clavius.
Protestant countries for a time refused to use the Gregorian Calendar.
England did not accept it until 1752, and Russia, which was 13 days behind the rest of the world, adopted it only recently.
New Catholic Dictionary