Yesterday was the Solemnity of the Body and Blood of Christ, also known by its Latin title, Corpus Christi. This is a celebration dear to my heart as Christ's Eucharistic Presence was a large part of what drew me to the Catholic Church.
As Fr. Reginald pointed out in his homily yesterday, this is an unusual feast day in that it honors a doctrine rather than an individual -- although of course the doctrine is very closely related to the Person of Christ. Catholics, along with Orthodox Christians, believe that the consecrated Bread and Wine are in fact the Body and Blood, Soul and Divinity of Jesus Christ substantially present in the Eucharist.
Why do we believe this? Because Jesus taught it. Because the Apostles and early Christians accepted it. Because the Church has always believed it. Because we have been given the gift of faith in His Real Presence.
Scripture on the Real Presence of Christ in the Eucharist
Early Church Fathers on Christ Present in the Eucharist