For the origin of Jewish Pentecost, consult the Torah here. The festival fifty days after Passover was a dedication of the first fruits of the agricultural activity of the ancient Israelites. (No wonder the claim the discples had drunk too much new wine.) The descent of the Holy Spirit may well be a first fruit of God’s grace in the era of evangelization and universal salvation.
I found Deuteronomy 16:12-15 interesting:
Remember that you too were once slaves in Egypt, and carry out these statutes carefully. You shall celebrate the feast of Booths for seven days, when you have gathered in the produce from your threshing floor and wine press. You shall make merry at your feast, together with your son and daughter, your male and female slave, and also the Levite, the alien, the orphan and the widow who belong to your community. For seven days you shall celebrate this pilgrim feast in honor of the Lord, your God, in the place which he chooses; since the Lord, your God, has blessed you in all your crops and in all your undertakings, you shall do nought but make merry.
A reminder, again, of the origins in slavery and destitution. Then a party for everyone.
A reminder from St Cyril of Jerusalem about our confirmation:
Be sure not to regard the chrism merely as ointment. Just as the bread of the eucharist after the invocation of the Holy Spirit is no longer just bread, but the body of Christ, so the holy chrism after the invocation is no longer ordinary ointment but Christ’s grace, which through the presence of the Holy Spirit instills his divinity into us.
May the Holy Spirit strengthen us all on this pilgrim way.