Shavuot ought to be the most popular of the major festivals. There are no long services, like Rosh Hashanah; no fasting as on Yom Kippur; no rain-spattered meals in a draughty succah. And a slice of cheesecake is a lot more appetising than a week-long diet of matzah.

It ought to be the most important festival, too, for there can be no more definitive moment for Judaism than the giving of the Torah. But the anniversary of Sinai which Shavuot commemorates is a product of rabbinic interpretation; it is not an idea found in the Bible. And here Shavuot is unique.

The observance of the other four festivals is in some way connected to their biblical origins, whether shaking a lulav, blowing a shofar or eating bitter herbs. But Shavuot’s explicit purpose as a harvest festival or Yom Habikkurim, the day of first fruits (as it is called in Numbers 28:26), has almost vanished from trace in the diaspora.

The decoration of the synagogue with flowers is about as far as we get to recalling the festival’s agricultural roots. There is a liturgical echo, too, in the reading of the book of Ruth, which is set during the harvest.

In ancient times, Shavuot marked the start of the season during which the people of Israel could bring a thanksgiving offering of first-fruits to the Temple: “You shall take from the first yield of all the fruit of the soil” (Deuteronomy 26:2) — which the rabbis understood to mean the seven species particularly associated with the land of Israel, barley, wheat, figs, olives, dates, grapes and pomegranates.

Ordinary folk would carry their produce in wicker baskets: according to the Mishnah, the containers of the rich would be overlaid with silver and gold. Flutes would escort the pilgrims as they arrived in Jerusalem, where they would be greeted by dignitaries from the Temple. Even King Agrippa was said to have borne his basket of fruit on his shoulder.

They would then recite the passage from Deuteronomy 26:5-9, which recalls Egypt and the Exodus. Its text is familiar to us because the first four verses are quoted in the Haggadah, beginning “A wandering Aramean was my father.” The declaration establishes a continuity between Pesach and Shavuot — from liberation from slavery to national independence in the Promised Land. Whereas the enslaved Israelites were once dependent on the Egyptians for their food, now they are responsible, under the watchful eye of heaven, for producing it themselves. The first fruits are a sign of blessing in a land “flowing with milk and honey”.

But there is something more. Immediately after the ceremony of thanksgiving for the first fruits, the Torah reminds the Israelites of their obligations to provide for the needy: “the Levite, the sojourner, the orphan and the widow, and they shall eat within your gates and be satisfied” (Deuteronomy 26:12). If the land is fertile, its fruits must be shared with the vulnerable and disadvantaged. And since the prayer over the first fruits recall the travails of Egypt, the Torah suggests that historical memory of hard times is a necessary prompt to social responsibility.

The duty to those in need is underlined in two biblical references to Shavuot. In the section about festivals in the recently read sidrah of Emor, the passage on Shavuot concludes with the direction to leave the corners of a reaped field “for the poor and sojourner” (Leviticus 23:22).

Then, in Deuteronomy 16:11, the commandment is given to rejoice on the Festival of Weeks — “you, your son and your daughter… with the sojourner and the orphan and the widow”. In other words, a festival is no festival unless everyone is included in its celebration.

Shavuot — which falls seven weeks and a day after Pesach, hence the name, Weeks — represents a two-fold journey: from deliverance from Egypt to acceptance of the Torah at Sinai: and then to settlement in the land of Israel, with its fruitful harvests. But its upbeat mood — “You shall rejoice” — is punctuated by one surprising turn in the liturgy.

The haftarah on the second day, which comes from the Book of Habakkuk, depicts an apocalyptic vision of God, rising up to “thresh” the nations in anger and bring salvation to His people. At the end the prophet declares: “Although the fig tree shall not blossom, neither shall fruit be in the vines; the labour of the olive shall fail…. Yet I will rejoice in the Lord.”

Instead of a gentle pastoral scene with the people enjoying the fruits of the land, Habakkuk draws a landscape of barren vines and blasted crops. For the prophets, prosperity may hold its dangers; people may become indulgent and moral decay set in. But spiritual resilience may be most needed in times of upheaval and adversity, when it can be hard to hang on to your ideals and hopes.