Shavuot, also known as the Feast of Weeks, because God’s people were to count seven weeks (49 days) after Passover until the 50th Day for the observance of Shavuot. Originally the 50th Day (Pentecost)  commemorated the giving of the Torah, GOD’s WORD, through Moses, which provided the Hebrews with instructions for living a holy lifestyle.

GIVING OF GOD’S WORD 

The Torah made the Hebrew people into a nation consecrated to Adonai. No other nation on earth was like God’s people. In Exodus chapters 19 and 20 we find the account of Adonai giving Moses the ten commandments.

Exodus 19: 1 In the Third Month after the people of Isra’el had left the land of Egypt, the same day they came to the Sinai Desert. 2 After setting out from Refidim and arriving at the Sinai Desert, they set up camp in the desert; there in front of the mountain, Isra’el set up camp. 3 Moshe went up to God, and ADONAI called to him from the mountain: “Here is what you are to say to the household of Ya’akov (Jacob), to tell the people of Isra’el: 4 ‘You have seen what I did to the Egyptians, and how I carried you on eagles’ wings and brought you to myself. 5 Now if you will pay careful attention to what I say and keep my covenant, then you will be my own treasure from among all the peoples, for all the earth is mine; 6 and you will be a kingdom of cohanim for me, a nation set apart.’These are the words you are to speak to the people of Isra’el.”7 Moshe came, summoned the leaders of the people and presented them with all these words which ADONAI had ordered him to say. 8 All the people answered as one, “Everything ADONAI has said, we will do.” Moshe reported the words of the people to ADONAI.

Exodus 20:1 Then God said all these words: 2 “I am ADONAI your God, who brought you out of the land of Egypt, out of the abode of slavery. 3 “You are to have no other gods before me.”

Exodus 34:22 22 “Observe the festival of Shavu’ot with the first-gathered produce of the wheat harvest, and the festival of ingathering at the turn of the year.

Deuteronomy 16:10 You are to observe the festival of Shavu‘ot [weeks] for Adonai your God with a voluntary offering, which you are to give in accordance with the degree to which Adonai your God has prospered you.

Pentecost is the Greek word for 50th Day aka Shavuot, which is one of the three commanded feasts. It also marks the beginning of Israel’s the summer wheat harvest, when every Hebrew adult male was commanded to bring a portion of their wheat harvest as an offering to God at the Temple in Jerusalem for Shavuot.

BIRTH OF THE MESSIAH

NOW it is during the month of Sivan, the Third Biblical Month, that Mary would have been in her ninth month of pregnancy. Mary and Joseph (Miriam and Yosef) would have been on their way to Bethlehem which is 5 miles south of Jerusalem, when she goes into labor with Yeshua/Jesus. To me it seems obvious why there was no room accommodations available, probably because “every Hebrew adult male was commanded to bring a portion of their wheat harvest as an offering to God at the Temple in Jerusalem,” for Shavuot. The bible also tells us the Roman Emperor called for a census, which politically makes perfect sense because all the Hebrew males were in town for Shavuot. In the gospel of Luke 2:1–5 we find sound evidence:
In those days a decree went out from Emperor Augustus that all the world should be registered. This was the first registration and was taken while Quirinius was governor of Syria. All went to their own towns to be registered. Joseph also went from the town of Nazareth in Galilee to Judea, to the city of David called Bethlehem, because he was descended from the house and family of David. He went to be registered with Mary, to whom he was engaged and who was expecting a child.

GIVING OF THE HOLY SPIRIT – PENTECOST

The giving of the Holy Spirit in the Upper Room is recorded in the book of Acts. The  followers of Yeshua/Jesus waited in the Upper Room in Jerusalem praying together and continuing to Count the Omer until the 50th day arrived. In the Book of Acts, we find important details of the story.
Acts 2:1–13 – Complete Jewish Bible
“1 The festival of Shavu‘ot arrived, and the believers all gathered together in one place. 2 Suddenly there came a sound from the sky like the roar of a violent wind, and it filled the whole house where they were sitting. 3 Then they saw what looked like tongues of fire, which separated and came to rest on each one of them. 4 They were all filled with the Ruach HaKodesh (Holy Spirit) and began to talk in different languages, as the Spirit enabled them to speak. 5 Now there were staying in Yerushalayim religious Jews from every nation under heaven. 6 When they heard this sound, a crowd gathered; they were confused, because each one heard the believers speaking in his own language. 7 Totally amazed, they asked, “How is this possible? Aren’t all these people who are speaking from the Galil? 8 How is it that we hear them speaking in our native languages? 9 We are Parthians, Medes, Elamites; residents of Mesopotamia, Y’hudah, Cappadocia, Pontus, Asia, 10 Phrygia, Pamphylia, Egypt, the parts of Libya near Cyrene; visitors from Rome; 11 Jews by birth and proselytes; Jews from Crete and from Arabia. . . ! How is it that we hear them speaking in our own languages (tongues) about the great things God has done?” 12 Amazed and confused, they all went on asking each other, “What can this mean?” 13 But others made fun of them and said, “They’ve just had too much wine!”
At these three times of the year all Israelites were commanded by GOD to come to the Temple (make a pilgrimage trip) to Jerusalem for worship and celebration. Adult males were not to come empty handed; each man was to present an offering to Adonai, and everyone was to rejoice before the Lord. This celebration commemorated freedom from physical Egyptian bondage and freedom from pagan idolatry because Israel was given enlightenment through HIS WORD.

CONCEPTION OF THE MESSIAH

Luke records very probably one of the most important details for us to calculate Yeshua/Jesus time of birth.
Luke 1:26-36 In the Sixth Month, the angel Gavri’el was sent by God to a city in the Galil called Natzeret, 27 to a virgin engaged to a man named Yosef, of the house of David; the virgin’s name was Miryam. 28 Approaching her, the angel said, “Shalom, favored lady! ADONAI is with you!” 29 She was deeply troubled by his words and wondered what kind of greeting this might be. 30 The angel said to her, “Don’t be afraid, Miryam, for you have found favor with God. 31 Look! You will become pregnant, you will give birth to a son, and you are to name him Yeshua. 32 He will be great, he will be called Son of Ha‘Elyon. ADONAI, God, will give him the throne of his forefather David; 33 and he will rule the House of Ya‘akov forever — there will be no end to his Kingdom.” 34 “How can this be,” asked Miryam of the angel, “since I am a virgin?” 35 The angel answered her, “The Ruach HaKodesh will come over you, the power of Ha‘Elyon will cover you. Therefore the holy child born to you will be called the Son of God. 36 “You have a relative, Elisheva (Elizabeth), who is an old woman; and everyone says she is barren. But she has conceived a son and is six months pregnant! 37 For with God, nothing is impossible.” 38 Miryam said, “I am the servant of ADONAI; may it happen to me as you have said.” Then the angel left her.