Contrary to popular belief, the Immaculate Conception celebrates the conception of the Virgin Mary and not her son, Jesus Christ. Christians and especially Catholics believe that on 8 December, Saint Anne, the Virgin Mary's mother, became pregnant with Mary through natural means.
According to the New Testament, God chose Mary from the moment of her conception to be the mother of Jesus and so Mary was, from the very first moment of her existence, pure and without sin.
The date (which was proclaimed in 1854 by Pope Pius IX) is exactly nine months before Mary's birth, on September 8th, which is also celebrated in Catholic countries as the Nativity of the Blessed Virgin Mary.
The date falls on Sunday - Monday is just a "bank holiday" and some shops will be open all or part of the day.