The Catholic Church first celebrated the feast of Our Lady of the Rosary in 1571 when the Christian military won a major victory over the Turkish. After the Turkish fleet, under the command of Selim II (1524-1574), had overrun Cyprus, Pope Saint Pius V (1504-1572) tried to start another crusade. However, every country in Europe refused to help except Spain.
King Philip II (1527-1598) placed his entire navy under the care of his half-brother Don Juan of Austria (1547-1578). It is quite likely that many considered this a bad decision. Juan was illegitimate and, because there was a dispute over whether or not he was of royal blood, he was almost constantly at odds with his family. Philip had wanted him to become a priest, but Juan refused and went into the military.
Juan prepared for battle by fasting and praying. On the evening of October 6, Pope Pius prayed the rosary with the entire city of Rome.
The Christian and Turkish fleets met in the Ionian Sea on October 7, 1571. The five hour long Battle of Lepanto ended with Selim being absolutely annihilated by Don Juan. When the Holy Father was informed supernaturally of the victory, he declared that October 7 would always be a Marian feast in thanksgiving.
Although the holy rosary is not the official prayer of the Church, it is the most popular Catholic devotion in existence. According to tradition, the first rosary was given to Saint Dominic (1170-1221) when Our Lady appeared to him in the year 1214. Dominic had recently founded the Order of Preachers, known today as the Dominicans, and Mary promised to help him if he spread devotion to the rosary.
During the 13th century, the Divine Office was the most common Catholic devotion. If prayed properly, the Divine Office would include all 150 psalms in one week. However, very few people other than the nobility, priests, and religious could read and it was, consequently, very difficult for the common man to practice this devotion. In order to mirror the 150 psalms, the original rosary contained 150 Hail Marys. Although the majority of people now pray rosaries with only 50 Hail Marys, the Dominicans still use the original 150 prayer rosary given to their founder.
October 7 can also be considered the saint day for people named Rosario or Rosaria.
Source: Carroll, Anne W. “Christ the King, Lord of History”