Many people mistakenly believe that this solemnity celebrates Jesus’ conception, but what is actually being celebrated is the immaculate conception of Mary. Jesus perfectly honored his mother by assuring that she be created immaculately or as Scripture says “full of grace,” that is conceived without sin, one of only four human beings to come into existence in this way (the other three are Adam, Eve, and Jesus).
This is one of the most important Marian feasts in the liturgical calendar, so much so that for dozens of countries this is a public holiday and the occasion is celebrated with parades, fireworks, processions, food, and cultural festivities in honor of the Blessed Virgin Mary. The Church has proclaimed this occasion to be a Holy Day of Obligation meaning that all Catholics are obliged out of obedience to go to Mass on this day, unless a person has a serious reason why he cannot go. Otherwise deliberately forgoing this obligation is a grave sin.