By the giving of the Law on Mount Sinai, the covenant between God and His chosen people received an actual outward sign; in like manner the covenant of the New Law received its consummation through the coming of the Holy Ghost. From that time on began its development and propagation; therefore the Feast of Pentecost is justly styled the birthday of our Holy Church.
2. The Feast of Pentecost introduced and celebrated by the Apostles was always considered one of the greatest feasts of the Ecclesiastical Year. It admonishes us to keep the Ten Commandments faithfully; for these Commandments are not to cease in the New Law, but rather to be performed in a more perfect manner. It reminds us of the descent of the Holy Ghost, in which we became partakers by the Sacrament of Confirmation. On Pentecost we should specially implore the Holy Ghost to renew in our souls the graces of this sacrament. On this day the Holy Ghost assumed supreme guidance of the Church, and became the dispenser of divine grace, therefore every faithful child should on this feast offer to God a heart overflowing with gratitude for the priceless gift of faith, as well as for the numberless graces and henefits received through the Church.
Source: The Ecclesiastical Year, Imprimatur 1903