The month of July is dedicated to the Most Precious Blood of Jesus. Our Lord's blood poured out on the Cross purchased our salvation, washed clean the robes of the martyrs, and gave birth to the Church as it flowed from his wounded side. The Precious Blood of Christ — now pulsing through his Mystical Body — continues its salvific work, preserving and purifying, repairing and providing nourishment for regeneration and renewal of its members. Therefore the Church developed a devotion to Jesus' physical blood and its mystical power, just as it did for his Sacred Heart from which his blood poured out on the Cross.
The ancient Israelites considered blood to be both sacred as well as polluting. It was an essential part of their rituals but was also regarded as something that could cause ritual impurity if not properly dealt with. In Christianity, the blood of Jesus as well as the blood of the martyrs was considered redemptive. Just as the animals of ancient Jewish ritual washed away the sins of the Israelites, the blood of Jesus has washed away our sins. The blood of the martyrs watered the seed of Christianity and gave birth to the Church.
Many saints had a devotion to the Precious Blood of Jesus, most notably St. Catherine of Siena. Devotion to the Precious Blood spread greatly through the preaching of Bl. Gaspar del Bufalo, a 19th century priest and founder of the Missionaries of the Precious Blood, and was later approved and recommended by the Holy See.
In the pre-Vatican II Church, July 1 was celebrated as the Feast of the Most Precious Blood of Jesus. In the post-conciliar liturgical calendar, this feast was combined with that of Corpus Christi (the Body of Christ) which is celebrated on the Sunday after Trinity Sunday in the United States. In much of the world, the Solemnity of the Body and Blood of Christ is celebrated on the Thursday following Trinity Sunday and is considered a Holy Day of Obligation.