140 Public and common prayer offered to God by all at the same time was customary in antiquity only on certain days and at certain times. Indeed, people prayed to God not only in groups but in private houses and occasionally with neighbors and friends. But soon in different parts of the Christian world the practice arose of setting aside special times for praying, as for example, the last hour of the day when evening set in and the lamps were lighted; or the first, heralded, when the night was coming to an end, by the crowing of the cock and the rising of the morning star. Other times of the day, as being more suitable for prayer are indicated in Sacred Scripture, in Hebrew customs or in keeping with the practice of every-day life. According to the acts of the Apostles, the disciples of Jesus Christ all came together to pray at the third hour, when they were all filled with the Holy Ghost; and before eating, the Prince of the Apostles went up to the higher parts of the house to pray, about the sixth hour; Peter and John "went up into the Temple at the ninth hour of prayer" and at "midnight Paul and Silas praying . . . praised God."
Source: Mediator Dei (Vatican.va)