The Jewish Pentecost & the Church's Pentecost

This comparison by Dom Prosper Guéranger in his The Liturgical Year is helpful in placing the coming feast in its Scriptural and historic context. These paragraph are just the beginning of a much lengthier (5670 word) essay which is reproduced halfway down this webpage.

In the old and figurative Law, God foreshadowed the glory that was to belong, at a future period, to the fiftieth day. Israel had passed the waters of the Red Sea, thanks to the protecting power of his Paschal Lamb! Seven weeks were spent in the desert, which was to lead to the promised land; and the very morrow of those seven weeks was the day whereon was made the alliance between God and His people. The Pentecost (the fiftieth day) was honoured by the promulgation of the ten commandments of the divine law; and every following year, the Israel­ites celebrated the great event by a solemn festival. But their Pentecost was figurative, like their Pasch: there was to be a second Pentecost for all people, as there was to be a second Pasch, for the Redemption of the whole world. The Pasch, with all its triumph­ant joys, belongs to the Son of God, the Conqueror of death: Pentecost belongs to the Holy Ghost, for it is the day whereon He began His mission into this world, which, henceforward, was to be under His Law.

But how different are the two Pentecosts! The one, on the rugged rocks of Arabia, amidst thunder and lightning, promulgates a Law that is written on tab­lets of stone; the second is in Jerusalem, on which God's anger has not as yet been manifested, because it still contains within its walls the first fruits of that new people, over whom the Spirit of love is to reign. In this second Pentecost, the heavens are not overcast, nor is the roar of thunder heard; the hearts of men are not stricken with fear, as when God spake on Sinai; repentance and gratitude are the sentiments now uppermost. A divine fire burns within their souls, and will spread throughout the whole world. Our Lord Jesus had said: 'I am come to cast fire on the earth; and what will I, but that it be kindled?' The hour for the fulfilment of this word has come: the Spirit of love, the Holy Ghost, the eternal uncreated Flame, is about to descend from heaven, and realize the merciful design of our Redeemer.

Jerusalem is filled with pilgrims, who have flocked thither from every country of the Gentile world. They feel a strange mysterious expectation working in their souls. They are Jews, and have come from every foreign land where Israel has founded a syna­gogue; they have come to keep the feasts of Pasch and Pentecost. Asia, Africa, and even Rome, have here their representatives. Amidst these Jews pro­perly so called, are to be seen many Gentiles, who, from a desire to serve God more faithfully, have embraced the Mosaic law and observances; they are called proselytes. This influx of strangers, who have come to Jerusalem out of a desire to observe the Law, gives the city a Babel-like appearance, for each nation has its own language. They are not, however, under the influence of pride and prejudice, as are the inhabitants of Judea; neither have they, like these latter, known and rejected the Messias, nor blasphemed His works whereby He gave testimony of His divine character. It may be that they took part with the other Jews in clamouring for Jesus' death; but they were led to it by the chief priests and magistrates of the Jerusalem which they reve­renced as the holy city of God, and to which nothing but religious motives have brought them.