The Solemnity of the Immaculate Conception, also called Immaculate Conception Day, celebrates the sinless lifespan and Immaculate Conception of the Blessed Virgin Mary on 8 December, nine months before the feast of the Nativity of Mary, celebrated on 8 September. It is one of the most important Marian feasts in the liturgical calendar of the Roman Catholic Church celebrated worldwide.

By Pontifical decree, it is the patronal feast day of Argentina, Brazil, Chile, Italy, Korea, Nicaragua, Paraguay, the Philippines, Spain, the United States, and Uruguay. By royal decree, it is designated as the day honoring the patroness of Portugal. It is celebrated by the Roman Catholic Church and some select Protestant Christian denominations.

Since 1953, the Pope visits the Column of the Immaculate Conception in the Piazza di Spagna to offer expiatory prayers commemorating the solemn event.

The feast was first solemnized as a Holy day of obligation on 6 December 1708 under the Papal Bull Commissi Nobis Divinitus by Pope Clement XI and is often celebrated with Catholic Mass, parades, fireworks, processions, food, and cultural festivities in honor of the Blessed Virgin Mary and is generally considered a Family Day, especially in many pious Catholic countries.