His proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years.
Lilius also produced an original and practical scheme for adjusting the epacts of the moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.
In addition to the change in the mean length of the calendar year from 365.25 days (365 days 6 hours) to 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year, the Gregorian calendar reform also dealt with the accumulated difference between these lengths.
The canonical Easter tables were devised at the end of the third century, when the vernal equinox fell either on 20 March or 21 March depending on the year's position in the leap year cycle.
As the rule was that the full moon preceding Easter was not to precede the equinox, the date was fixed at 21 March for computational purposes and the earliest date for Easter was fixed at 22 March.
amounting to a 0.002% correction in the length of the year.
The motivation for the reform was to stop the drift of the calendar with respect to the equinoxes and solstices—particularly the vernal equinox, which set the date for Easter celebrations.
Transition to the Gregorian calendar would restore the holiday to the time of the year in which it was celebrated when introduced by the early Church.
The reform was adopted initially by the Catholic countries of Europe.
Protestants and Eastern Orthodox countries continued to use the traditional Julian calendar and adopted the Gregorian reform after a time, for the sake of convenience in international trade.
The last European country to adopt the reform was Greece, in 1923.
The Gregorian reform contained two parts: a reform of the Julian calendar as used prior to Pope Gregory XIII's time and a reform of the lunar cycle used by the Church, with the Julian calendar, to calculate the date of Easter.
The reform was a modification of a proposal made by Aloysius Lilius.