On this, the Feast of the Ascension of Our Lord, I was going to write a treatise against the transfer of feasts, but Eric Sammons beat me to it. Check out his entry.
Meanwhile, from the 1945 Marian Missal:
Forty days after the Resurrection, Our Lord Jesus Christ, attended by Angels, ascended into Heaven, in the sight of His most holy Mother Mary, His Apostles, and disciples, to the great wonder of them all. He entered into possession of the Kingdom of Heaven which He had gained by His sufferings, and St. Paul declares that God “hath made us sit together in the heavenly places, through Christ Jesus.” “There where the Head has gone, the Body is called to follow!” The liturgical festival of the Ascension, whilst less ancient than that of Pentecost, is one of the oldest of the cycle, and was already so universally observed that St. Augustine was able to attribute its institution to the Apostles themselves. At Rome, the Pope, after the night Office was concluded, and after Mass had been celebrated at the altar of St. Peter, was crowned by the cardinals, and towards the sixth hour was accompanied by bishops and clergy to the Lateran.
On this day Jesus was taken up into Heaven; His faithful disciples continued to gaze heavenward. The contemplative vision of Paradise is reserved, however, for those who have already passed into the Church Triumphant. The supreme glorification of the Head who is this day enthroned at the right hand of the Father in Heaven affects the members also, like the precious balsam which, as the psalmist tells us, descended from the head of Aaron on to his flowing beard and on to his gorgeous pontifical vestments. This spiritual unction is the gift of the Holy Ghost, which Christ obtains to-day from Heaven for his Church. Hence the connection between the Ascension and Pentecost is very close, nor can we understand the one without the other.