The Gregorian calendar corrected a major error in the existing Julian calendar, which Julius Caesar introduced in 46 B.C. The Julian calendar was 365 1/4 days long and the actual solar year was 365.2422 days. This meant that the Julian calendar exceeded the solar year by eleven minutes and fourteen seconds each year. This difference grew with each successive century, and by the late sixteenth century, the Julian calendar was ten full days longer than the solar calendar.
The Council of Trent (1545-1563) recognized that this growing deviation affected the liturgical calendar of the Catholic Church. Religious feast days no longer conformed to the guidelines established by the Council of Nicaea, 325 AD. For example, Easter, intended as a spring observance, would ultimately occur in the summer.
Pope Gregory XIII (1502-1585), elected in 1572, organized the necessary reform of the calendar. In 1577, he formed an international commission of distinguished experts to determine the necessary corrections. The commission approved a calendar worked-out by Luigi Lilius (d. 1576), a Neapolitan astronomer who had discovered that the Julian Calendar was ten days too long. In 1579, the pope ordered the construction of the first astronomical observatory at the Vatican. Here the commission completed the final details of calendar reform, including a more accurate lunar almanac. These details were largely the work of the German Jesuit Christopher Clavius (1537?-1612), a noted astronomer and mathematician.
Papal edict proclaimed the new Gregorian Calendar in February of 1582. This edict declared that the day after Thursday, October 4, 1582, would be Friday, October 15, thus dropping ten days and bringing the calendar in line with the solar year. The pope also approved an important reform involving leap years. Every fourth year would continue as a leap year, with an extra day in February. However, years ending in two zeroes would be leap years only if divisible by 400. In this manner, three days dropped every four centuries, thus avoiding major deviation from the solar year.
The stamps, vertical in format, measure 30 x 40 mm and have a perforation of 13 1/4 x 14. Along the top of each stamp appear the inscription "1582-1982" and the denomination. The words POSTE VATICANE appear along the bottom. The stamps were produced on white paper and printed in recess in sheets of forty. The Polygraphic Institute and Mint of the Italian State printed 900,000 complete .
"Calendar Reform." Vatican Notes 31, no. 4 (January 1983): 1, 8.