Pentecost (literally the fiftieth day) is the beginning of the Jewish festival of Shavuot. This is the religious festival that was all about celebrating God giving the law (torah) to the people of Israel (which coincidentally happened fifty days after the Exodus).
If you remember your flannelgraph correctly, you may remember seeing a fire at the top of the mountain. This passage (along with Ezekiel 1-2) is one of the theophanies (visible manifestation of God) that the Jewish people read during the feast of Shavuot. The other main reading for this Festival comes from the book of Ruth. That is interesting because the book of Ruth foreshadows the incorporation of the Gentiles into the people of God as she is not a Jew but a Moabite (who are forbidden in Deuteronomy 23:3 to be part of the Jewish people... they are to be totally separate).
What a beautiful setting for the giving of the Holy Spirit! The festival celebrates the moment when all the people saw a fire on top of the mountain and received the Law that was to help them cover their sin and live in right relation to God once again. Now, through the sacrifice of Jesus Christ, the fire is no longer on the mountain, but separates and rests on them individually. It fills them and they cannot help but cry out! The law that is being celebrated in Shavuot is being fulfilled before their very eyes.