What happened on this Pentecost day as is described in Acts 2:1-4:
And when the day of Pentecost was fully come, they were all with one accord in one place. And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. And they were all filled with the Holy Spirit, and began to speak with other tongues, as the Spirit gave them utterance, Acts 2:1-4.
In Old Testament times the Holy Spirit would be given to someone for a specific purpose, and when the deed was done the Holy Spirit would be removed. Some examples are: (1) Bezaleel, Ex 31:1-11, artisan for helping to build artwork for the Tabernacle; (2) Samson, Judges 14:5-6, killing of a lion; (3) Saul, 1 Sam 10:6, prophesy; and (4) Jephthah, Judges 11:29-32, winning a battle.
In New Testament times, Yahshua received Yahweh's Spirit, first to be tempted by Satan.
And Yahshua, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of Yahweh descending like a dove, and lighting upon him, Matt 3:16.
Then was Yahshua led up of the Spirit into the wilderness to be tempted of the devil, Matt 4:1.
When one is baptized and the elders lay hands on them and pray, Scriptures tell that they shall receive the Holy Spirit.
Then Peter said unto them, Repent, and be baptized every one of you in the name of Yahshua Messiah for the remission of sins, and ye shall receive the gift of the Holy Spirit, Acts 2:38.
Now when the apostles which were at Jerusalem heard that Samaria had received the word of Yahweh, they sent unto them Peter and John: Who, when they were come down, prayed for them, that they might receive the Holy Spirit: (For as yet he was fallen upon none of them: only they were baptized in the name of the Master Yahshua.) Then laid they their hands on them, and they received the Holy Spirit, Acts 8:14-17.
What we have seen is when Pentecost was made a high day in the list of high days, how that day was found in the Old Testament and in the New Testament, and what it should mean to us today.