Villa Mondragone and the Gregorian calendar

Every time you check the date on your phone you are using a calendar named after a pope, and according to tradition it was born on a hill in Monte Porzio Catone. The hill is the one of Villa Mondragone, the villa that dominates the road between the town and Frascati, with its long pale facade turned towards Rome.

What is Villa Mondragone?

It is one of the largest of the Tusculan Villas, the summer residences that families of the papal court built on the hills between Frascati and Monte Porzio from the sixteenth century onwards. It stands in Monte Porzio Catone, at Via Frascati 51, in a park of about 18 hectares, over the remains of a Roman villa attributed to the Quintilii family.

Since 1981 it has belonged to the University of Rome Tor Vergata, which uses it as a conference centre.

Who built it, and why “Mondragone”?

The villa was commissioned by Cardinal Marco Sittico Altemps, nephew of Pope Pius IV. The first core was built in 1573 and 1574 to a design by Martino Longhi the Elder, the architect of Palazzo Altemps in Rome.

Altemps wanted it as a summer home for Pope Gregory XIII, born Ugo Boncompagni, and his court. The name comes from him: the Boncompagni coat of arms shows a winged dragon, and the dragon returns again and again in the decoration of the palace and the garden. Mondragone means the dragon’s mountain.

Was the Gregorian calendar really born here?

It is the villa’s most famous tradition, and it is reported by Tor Vergata, by FAI, the Italian National Trust, and by the European Physical Society: in 1582 Gregory XIII, staying at Mondragone, issued here the bull Inter gravissimas, which reformed the Julian calendar used since the days of Julius Caesar.

The document is dated 24 February 1582. Sources use slightly different words, some say it was signed at the villa, others that it was promulgated there, but all of them recognise the link between the villa and the reform.

What did the reform change?

The Julian calendar was a little longer than the solar year, and over sixteen centuries it had drifted about ten days behind the seasons. The bull removed them at a stroke: in the countries that adopted it straight away, Thursday 4 October 1582 was followed by Friday 15 October.

To stop the error coming back, the leap year rule changed too: century years are leap years only if they divide by 400. That is why 2000 was a leap year and 1900 was not.

What happened after Gregory XIII?

• 1613. Cardinal Scipione Borghese, nephew of Pope Paul V, bought the villa together with the other Altemps estates. Under the Borghese, Mondragone lived its finest years: it was enlarged by the Dutch architect Jan van Santen, known in Italy as Giovanni Vasanzio, and held part of the family’s art and antiquities.
• 1626. Under Pope Urban VIII the papal summer residence moved to Castel Gandolfo, and the villa began a long decline.
• 1858. The writer George Sand stayed at the villa and set her novel La Daniella here.
• From 1865 or 1896. The Jesuits turned it into the Nobile Collegio Mondragone, a boarding school for the sons of noble and wealthy families, which ran until 1953. Sources disagree on the date: Wikipedia gives 1865, while Tor Vergata says the villa was sold to the Society of Jesus in 1896.
• 1981. The villa and its park passed to the University of Tor Vergata.

Galileo, Marconi and a mysterious manuscript

Mondragone also has a scientific history that few people know. According to the European Physical Society, in 1611 Galileo Galilei showed his astronomical discoveries with the telescope here, and in 1932 Guglielmo Marconi carried out pioneering microwave experiments at the villa, which led to the first microwave telephone link between the Vatican and Castel Gandolfo. For this reason, on 9 June 2025 the villa was named an EPS Historic Site of European physics.

In 2010 the Physics Department of Tor Vergata installed a Foucault pendulum in the library, which FAI describes as the tallest in central Italy.

Then there is the mystery. In 1912 the antiquarian bookseller Wilfrid Voynich bought a group of about thirty manuscripts from the Jesuits of Mondragone. One of them is the famous Voynich manuscript, an illustrated book written in a language or code that nobody has yet managed to decipher.

Can you visit Villa Mondragone?

The villa is a university building and conference centre, so it is not open every day like a museum. You can see it during events, conferences and special openings, such as those organised by FAI, when visitors enter rooms like the Retirata, the chapels, the Hall of the Caryatids and the Red Hall. Up to date information is on the Tor Vergata website.

It is worth stopping even from outside: from the forecourt the view opens over the whole Roman countryside.

In short

• Where: Via Frascati 51, Monte Porzio Catone, on the road to Frascati.
• Built: 1573 and 1574, for Cardinal Marco Sittico Altemps, designed by Martino Longhi the Elder.
• The calendar: in 1582 Gregory XIII issued the bull Inter gravissimas here; 4 October 1582 was followed by 15 October.
• Today: owned by Tor Vergata since 1981; a historic site of European physics since 2025.
• Nearby: the ruins of Tusculum, Cicero at Tusculum, Frascati wine.