The Bible normally uses the term heart to describe the center of a person, or the person itself. Thus Jesus says: Do not let your hearts be troubled (John 14:1) and your hearts will rejoice and no one will take your joy away from you (John 16:22). Expressions about the Heart of Jesus are absent from the Bible, but they grow from a particular devotional background and mysticism in the Middle Ages (13/14th cent.: Mechtild from Magdeburg, Gertrud from Hefta, Heinrich Seuse): mystical experience with Christ, devotion to the passion and death of Jesus (wounds from nails and lance, the crown of thorns). The devotion of the Sacred Heart focused on the heart pierced by a lance after the crucifixion (John 19:34). Thus the Heart of Jesus became a designation for his sacrificial suffering and his love that unites God and humanity. This gave rise to the notion of reparation for the insults of people who turn the divine love down.
The devotion of the Sacred Heart was especially fostered by the Jesuit Order and the Oratorian Order in the 16/17th centuries. The desire for a particular feast became more vivid in the second half of the 17th cent., promoted also by the visions of Margarita Maria Alacoque (1673-1675).
The feast was first celebrated by the Oratorian priest John Eudes in 1672. Eventually, in the 18th century, it spread across France, Italy, and Germany. It received its first official recognition by pope Clemens III in 1765 (only in Poland and certain confraternities in Rome) and Pius IX made it obligatory for the universal Church in 1856. Leo XIII consecrated the whole world to the Sacred Heart in 1899.
Liturgical texts make reference to the pierced heart as a fount of heavenly gifts. It is Heart wounded by our sins, but God grants the mercy of conversion and enables people to brotherly love.
…Jesus raised up high on the Cross gave himself up for us with a wonderful love and poured out blood and water from his pierced side, the wellspring of the Church’s Sacraments…Preface
Besides the solemnity, the devotion to the Sacred Heart is also expressed on the first Friday of each month and / or a holy hour the night before.