The Feast of the Immaculate Conception of Mary is one of the few Holy days of obligation on the Church calendar -- that is, all Catholics are obligated to attend Mass on this day. It is also a public holiday in countries where Catholicism is predominant.

The doctrine of the Immaculate Conception did not become official Catholic dogma until 1854, but it had been passionately held by many Catholics for centuries.

The Feast of the Immaculate Conception is celebrated on December 8, nine months before the feast of the Nativity of Mary, which is celebrated on September 8.