The festival of Shavuot falls this year on Friday 11 and Saturday 12 June 2027 (eve Thursday 10 June).
Service times for Shavuot
“You shall observe the Feast of the Eternal, your G-d, through voluntary offerings that you shall present in proportion to the blessings you will have received from the Eternal your G-d” (Deuteronomy 16, 19).
It is celebrated 50 days after Pesach and constitutes one of the three pilgrimage festivals (along with Sukkoth and Pesach).
It is the festival of offerings par excellence, also called Yom ha-bikkurim, “day of the first fruits”, because it was the day when people went from all over the country to the Temple in Jerusalem to offer the first fruits of the fields to the Sanctuary.
During all three pilgrimage festivals, the male population of Israel – though for Sukkoth the pilgrimage was sometimes expected of women and children as well (Exodus 2) – set out from every town, from every village, to bring their offering to the Temple in Jerusalem, as a testimony of their presence and their faithfulness to the divine command.
At the origins of the festival
Following the destruction of the second temple (70 of the Christian era), the festivity re-centres on the commemoration of the Covenant at Sinai, the gift of the Torah and the Ten Commandments.
Following the Exodus from Egypt, the children of Israel headed towards the land of Canaan, and seven weeks later they arrived before Mount Sinai where they received the command to wash their garments. On the third day, amidst lightning and thunder, the Lord spoke to the people who, however, faced with the unfolding of nature and the power of God’s voice, were seized with great terror.
Moses then received the command from G-d to go alone to the top of the mountain, where he remained forty days and forty nights to receive the two tablets of the Law, or more accurately, as the Hebrew text says, “the two Tablets of the Covenant”, the Decalogue.
The commonly used expression, Ten Commandments, is imprecise since the term found in the Torah, “assereth ha-dibberoth”, the “ten words”, assigns to the latter the value, rather, of messages.
But the people, seeing that Moses had not yet returned after forty days, feared they had been abandoned and, with the gold received as a gift from the Egyptians, built themselves a golden calf in imitation of the Apis bull adored by the latter.
Moses, coming down from Sinai, seeing the people indulging in the worship of an idol, smashed the Tablets of the Covenant considering them unworthy to receive them.
But after the transgressors were punished and the people repented of the sin they had committed, God, at the prayers of Moses, announced His forgiveness with the expression “Salachtì”: I have forgiven. And Moses, climbing Mount Sinai once again, received the Tablets of the Covenant anew. (1)
The liturgy
During the morning service, the parashah containing the Decalogue is read. During the day, the Book of Ruth is also read, which is connected to the harvest festival. The story of Ruth is very beautiful and poetic. Naomi and her two sons emigrate to Moab because of a famine. The sons marry two Moabite women, but both die. Naomi then decides to return to her land and Ruth, one of the daughters-in-law, goes with her because, she says, “your land is my land, your God is my God”. Urged on by her mother-in-law herself, Ruth marries Boaz, a wealthy landowner and distant relative of the family. According to tradition, Ruth is the ancestress of King David, from whose lineage the Messiah will descend. (1)
Customs
The festival of Shavuot has no special commandments. However, there are minhagim (customs) that have become established over the centuries (see How does the Minhag originate? in the Culture section of the website).
Sources: 
Synagogues are decorated with flowers and plants to remember that we are in the season of the first fruits, with a strong reminder of the ceremonies of offering first fruits at the time of the Temple.
There is also the custom of gathering on the night of Shavuot to study the Torah until dawn. This study, called Tikkun (repair), is meant to make up for the weakness of those who did not have the strength to stay awake when the Eternal One gave the gift of the Torah at Sinai. But this vigil consists first and foremost in waiting for the hour when the ancestors of Israel received the divine words. The origin of this custom is to be sought in sixteenth-century Kabbalah. The goal is to relive the experience of Sinai in fire and joy.(2)
(1) Ziv, bulletin de la commission judaisme de la Cté des Béatitudes (France)
(2) Le pietre del tempo, il popolo ebraico e le sue feste by Clara and Elia Kopciowski