WHAT makes a Jewish man smile on Friday evenings?
The tranquillity of the sabbath, when families gather around the dining table for a traditional meal, to be sure. The candle lighting. The blessings. But there is something else: Perhaps God's double mitzvah - literally, double "commandment" - that a man make love to his wife not just that night, but all throughout the next day. It may be the holy day's most important precept.
"This is an expression of moral values," says Lubavitch Rabbi Mendel Deren, who is based in the Old City of Jerusalem. "And by moral values, I mean that a man's duty to make love requires him to think not about what his own body wants, but the responsibility he has to care about his wife and her feelings. It's an act of love."
Such are the holy duties that befall an orthodox Jewish man week in, week out.
On this sabbath, today, the 15th day of the Hebrew month of Av, the lovemaking commandment has particular salience as Jews around the world observe the religious holiday known as Tu Ba'av, the Day of Love.
In Israel, Tu Ba'av has become a secular feast of amore - a day of love-related festivities. Israelis give cards and flowers to their loved ones and the date is one of the most popular for weddings. Bars and restaurants have been booked out for weeks as couples prepare for a romantic evening out. Barmen supposedly concoct aphrodisiac potions.
Rabbi Deren says the Tu Ba'av holiday was traditionally a day when unmarried men and women would take to the streets of Jerusalem to find a life partner. In ancient times women all wore simple white dresses so prospective husbands couldn't tell who was rich and who was poor.
"Not just the search for someone you take a liking to, this is the search for the person you can pair up with and then go out and take on the world, the partner who really fits you," Rabbi Deren says.
Until the early 1960s, Tu Ba'av remained a day strictly within the province of observant religious Jews.
"Then the hippies took over," says Mordechai Beck, a British-born teacher of rabbinic literature in southern Jerusalem. "For secular Israelis, this has become a day to celebrate, with parties, music, dancing. This has upset many of the rabbis, who believe that to celebrate in this way is a misreading of the texts.
"But I take a different view," Beck adds. "I think this is a day when people are proud to say they are Jewish, whether they are secular or not, and this is something I'm proud of."
The Jewish vision of sex is wider-ranging than the biblical instruction to be fruitful and multiply. Most intriguing, possibly, to non-Jews is the Jewish man's obligation, enshrined in the marriage contract, that he sexually satisfy his wife. A wife's sexual dissatisfaction, Beck says, is grounds for divorce.
Mr Beck, 63, and the father of four daughters, says: "Jewish attitudes towards sex are very different to Christianity's. We believe that we should enjoy all of the fruits that God gives us. The Christian view of sex leans more towards repressing those feelings.
"I think we encourage a more open view of sex within marriage than Christians."
Rabbi Deren, who is American-born, says: "The Christian view seems to be that sex is an unfortunate fact of life. But for Jews this is an opportunity that God gives every couple. So in our view, we should try to use it as an expression of deep love. Lovemaking can be coarse and animalistic, but do we stay away from it because of that? No! What it means is that next time, we try to do it better.
"For Jews, sex is not unfortunate," he said, in what may be the most joyfully agreed-upon truism across the Israeli secular-religious divide. "For us it is a fact of life and we have to be able to prove that we are worthy of expressing those moral values."
In Tel Aviv and Jerusalem tonight, the young will undulate to the thump of end-of-summer DJ's mixes and drinking blood-red love-day cocktails. It may be hard for the untrained eye to discern a spiritual aspiration in this. Yet Rabbi Deren says: "You'd be surprised how the body falls into line, and will you become better? Yes. Absolutely, yes. It is a long process, but every young couple has the hope of a long married life together."