“We declare, we pronounce, and we define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular privilege and grace granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.” 
The 8th of December is an important day - the Feast of the Immaculate Conception. It is also the day on which in 1947 Our Blessed Mother gave us the Hour of Grace.
Our Lady said as follows when she appeared to Sister Pierina in a little church in Montichiari, Italy: "It is my wish that every year, on 8th December, at noon, the HOUR OF GRACE FOR THE WORLD be celebrated. Many divine and bodily graces will be received through this devotion. Our Lord, my Divine Son Jesus, will send His overflowing mercy if good people will pray continuously for their sinful brother.... it is my wish that the HOUR OF GRACE FOR THE WORLD be made known and spread throughout the world. If anyone is unable to visit his church, yet will pray at home, he will also receive graces through me.... will find a secure heavenly ladder and receive protection and grace through my motherly heart."
THE REQUEST OF OUR BLESSED MOTHER FOR THE HOUR OF GRACE:
Our Lady promised she will hear any prayer we make at this time.
1. Day and time of the Hour of Grace: December 8th, Feast of the Immaculate Conception, to be started at 12:00 noon and continuing until 1:00 p.m. for one full hour of prayer.
2. During this hour, the person making the Hour of Grace either at home or at church must put away all distractions (do not answer the telephones, or answer any doors, or do anything but totally concentrate on your union with God during this special Hour of Grace).
3. Begin the Hour of Grace by praying three times the 51st Psalm with out-stretched arms. (Psalm 51 appears below).
4. The rest of the Hour of Grace may be spent in silent communication with God meditating upon the Passion of Jesus, saying the Holy Rosary, praising God in your own way, or by using favourite prayers, singing hymns, meditating upon other psalms, etc.
Psalm 51
1 Have mercy on me, O God, according to Your unfailing Love; according to Your great Compassion blot out my transgressions.
2 Wash away all my iniquity and cleanse me from my sin.
3 For I know my transgressions, and my sin is always before me.
4 Against You, You only, have I sinned and done what is evil in Your Sight; so You are right in Your verdict and justified when You judge.
5 Surely I was sinful at birth, sinful from the time my mother conceived me.
6 Yet You desired faithfulness even in the womb; You taught me wisdom in that secret place.
7 Cleanse me with hyssop, and I will be clean; wash me, and I will be whiter than snow.
8 Let me hear joy and gladness; let the bones You have crushed rejoice.
9 Hide Your Face from my sins and blot out all my iniquity.
10 Create in me a pure heart, O God, and renew a steadfast spirit within me.
11 Do not cast me from Your Presence or take Your Holy Spirit from me.
12 Restore to me the joy of Your Salvation and grant me a willing spirit, to sustain me.
13 Then I will teach transgressors Your Ways, so that sinners will turn back to You.
14 Deliver me from the guilt of bloodshed, O God, You who are God my Savior, and my tongue will sing of Your righteousness.
15 Open my lips, Lord, and my mouth will declare Your praise.
16 You do not delight in sacrifice, or I would bring it; You do not take pleasure in burnt offerings.
17 My sacrifice, O God, is a broken spirit; a broken and contrite heart You, God, will not despise.
18 May it please You to prosper Zion, to build up the walls of Jerusalem.
19 Then You will delight in the sacrifices of the righteous, in burnt offerings offered whole; then bulls will be offered on Your altar.
