What are we celebrating?
The Feast Day marks the Immaculate Conception - that is, the conception of the Virgin Mary in the womb of her mother, Saint Anne.
It's often mistakenly thought to mark Mary's conception of Jesus, but actually marks the conception of Mary herself. Unlike Mary, Saint Anne became pregnant in the usual biological way, Catholics believe, but the conception was 'immaculate' because God intervened, absolving Mary of original sin.
According to Catholic dogma, all humans are born with original sin, which is why babies are baptized shortly after birth to make them worthy of entry to Heaven. But Mary was never tainted by original sin, kept 'immaculate' from the moment of her conception because God knew she would one day give birth to Jesus Christ.
December 8th was first officially declared a holy day by the Vatican in 1854, when Pope Pius IX settled disputes and confirmed that the conception had been immaculate. But the first celebrations of the event happened as early is the seventh century.
How is it marked?
The pope will lay a wreath at the foot of the 12-metre tall Colonna della Immacolata, by the statue of the Madonna (Mary) in Rome's Piazza Mignanelli, while members of the Italian fire service place another floral wreath on the arm of the statue.
There will be special masses held at Catholic churches across Italy.
There are plenty of other celebrations allowing you to enjoy the festive atmosphere, with parades, music and street entertainment taking place in towns across the country.
In Abruzzo it's traditional to celebrate around a bonfire, with fire symbolizing purity, fertility and love. Other places hold torchlit processions and firework displays.
Does this mean everything will be closed?
Because the holiday falls during Advent, many shops stay open to allow for Christmas shopping.
However, make sure you check transport before trying to go anywhere, as most bus and rail routes will be running on a limited service. As usual, government offices, post offices, banks and schools are closed for the public holiday, so it's not a good time to catch up on admin.
This article was first published in December 2016.