Feast of the Immaculate Conception
The Solemnity of the Immaculate Conception celebrates the Immaculate Conception of the Blessed Virgin Mary, on 8 December, nine months before the feast of the Nativity of Mary on 8 September. It is one of the most important Marian feasts in the liturgical calendar of the Latin Church.
Source: Wikipedia — Feast of the Immaculate Conception (CC BY-SA 4.0)