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