PENTECOST:  According to the Old Testament, you would go to the day of the celebration of Firstfruits, and beginning with that day, you would count off 50 days. The fiftieth day would be the Day of Pentecost. So Firstfruits is the beginning of the barley harvest and Pentecost is the celebration of the beginning of the wheat harvest.   

This is why the first verse says "and when the day of Pentecost was fully come" 

The mystery here that is prophectic is that 40 days after Jesus  Rose he Ascended up to Heaven, and 10-days later the Holy Spirit fell 40+10 = 50 PENTECOST

 Pentecost (properly, the fiftieth day after the Passover, which continues to tie to the prophecy of and Promise from Jesus of the Power from on high

1 And when the day of Pentecost was fully come, they were all with one accord in one place.

2 And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting.

3 And there appeared unto them cloven tongues like as of fire, and it sat upon each of them.

4 And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.

5 And there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven.

6 Now when this was noised abroad, the multitude came together, and were confounded, because that every man heard them speak in his own language.

7 And they were all amazed and marvelled, saying one to another, Behold, are not all these which speak Galilaeans?

8 And how hear we every man in our own tongue, wherein we were born?

12 And they were all amazed, and were in doubt, saying one to another, What meaneth this?

13 Others mocking said, These men are full of new wine.

14 But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judaea, and all ye that dwell at Jerusalem, be this known unto you, and hearken to my words:

15 For these are not drunken, as ye suppose, seeing it is but the third hour of the day.

16 But this is that which was spoken by the prophet Joel;

17 And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams:

18 And on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy:

19 And I will shew wonders in heaven above, and signs in the earth beneath; blood, and fire, and vapour of smoke:

20 The sun shall be turned into darkness, and the moon into blood, before the great and notable day of the Lord come:

21 And it shall come to pass, that whosoever shall call on the name of the Lord shall be saved.

36 Therefore let all the house of Israel know assuredly, that God hath made the same Jesus, whom ye have crucified, both Lord and Christ.

37 Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?

38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.

Just before Jesus ascended into heaven, He told His followers to stay in Jerusalem and wait for the Holy Spirit (Acts 1:4–5). The eleven remaining apostles, along with the women who were so important to Jesus' ministry, Jesus' mother Mary, Jesus' brothers, and dozens of others, settle into the upper room of a house to pray and confer (Acts 1:12–15). For about nine days, they have little to do but praise God (Luke 24:52–53) and choose a successor to Judas (Acts 1:12–26). It's likely they are in this room when Acts 2 opens.

"All together" is from the Greek root word homou, related to the term homothymadon, translated "one accord" in Acts 1:14. Literally, it may mean "together in a group," but spiritually it refers to many individuals working together harmoniously. This attitude will characterize the church in Jerusalem for years to come. They will face many challenges including persecution (Acts 8:1), opposition from within (Acts 11:1–18; 21:17–24), and the inclusion of Gentiles who do not have the Jewish background the founding apostles do (Acts 15:1–35). In all these things, the leaders of the church of Jerusalem strive to maintain a unified front as they seek guidance from the Holy Spirit.

"Pentecost" is derived from the Greek word for "fiftieth." It falls seven weeks and one day—fifty days—after the sabbath that occurs during the weeklong Feast of Unleavened Bread. Pentecost is also called the "Feast of Weeks" because that same period is seven weeks or "a week of weeks." After Passover, Pentecost is the next major feast for which Jews from all over the Roman Empire will come to Jerusalem. It is given in the Mosaic Law in Leviticus 23:15–21 and Deuteronomy 16:9–12.

The Feast of Weeks is the second of three feasts that include a firstfruits offering. When people came to Jerusalem for Pentecost, they were to bring the first of their grain as a free-will offering. On this Pentecost, Jerusalem offers a kind of free-will offering to God as three thousand people believe in Jesus (Acts 2:41).