Venture near a register office or a church on Saturday and you could find yourself dodging unusual numbers of little girls in big dresses. Not only have we reached the peak of the wedding season, but this year – happily for those who flog frocks and produce rubber chicken – the luckiest day of the century falls on a Saturday. Roll on 7.7.07, and prepare to shed a tear as record numbers of couples pledge their troth.

Thanks to the multicultural status of seven as a lucky number, the phenomenon is international. In America inquiries about 7.7 weddings began in 2005, and in Las Vegas, Chapel of the Flowers will host 113 ceremonies on Saturday instead of the usual 30 to 50. The wedding website theknot.com has a register of 38,000 weddings for 7.7, compared with a typical July Saturday norm of 12,000, and in Britain the date’s association with the London bombings has done little to dampen enthusiasm. Most register offices are fully booked: in Brighton and Hove 31 couples will marry, compared with 17 on the corresponding Saturday of last year, and in Richmond upon Thames there will be 19 ceremonies, compared with 4 last year.

But the question all this raises is why do people marry at all? There is no longer a social or cultural imperative to marry in the UK, there is no shame in couples living together or having children out of wedlock. A third of children are born to unmarried parents, and the number of marriages in England and Wales – 244,710 in 2005 – is the lowest since 1896. Why bother with the ceremony? Why, in 2005, did 98,580 couples in England and Wales wed for at least the second time? Who are these people?

In a cynical world accustomed to the ravages of divorce, the answers are reassuring. People marry, say those who study such things, because they want to express their love publicly (registrars believe that the motivation is the same for civil partnerships). Hankies ready, then, as Jenny North, Relate’s head of public policy, explains: “People don’t marry as part of the old-fashioned transaction, which would be that the woman buys financial security and the man buys children and a housekeeper. Today there’s no guarantee that marriage will lead to children or that the man will be the breadwinner.

“They now marry for explicitly romantic reasons to do with finding the right person, an everlasting love match and public commitment. The private commitment may have been made long ago, they probably already live together and may have children. It’s something they do when they have all their ducks in a row, when they’ve got good jobs, bought the house. Marriage used to be something you did on your way to maturity; now it’s something you do when you’ve achieved maturity.”

This is why those who marry are getting older: the average age of first-time brides is 29, bridegrooms, 31. North says that they also tend to be middle class because of the expectation that weddings must be special and therefore expensive (the average cost is now put at £18,000). “It’s a sign that they’ve made it. The young and the poor are not marrying in such great numbers because they can’t afford it.”

Penny Mansfield, the director of One Plus One – an organisation that researches family relationships – confirms that the married population is becoming more homogenous. “Married people are more likely to be better educated and from stable family backgrounds where marriage and its association with stability might be attractive to them. They’re from the higher social class groups and they have better jobs. This is why married people do better – there is this selection within marriage. It still has legal advantages. If you have assets, you will be better protected within marriage, if you are concererned about being prudent, marriage will be important to you.

“People also like the explicit symbolism in the company of family and friends. You tend to get more support from your family if they feel clear about the status of your relationship. You’re another link in the chain. Marriage is more premeditated now, but people still value it. They talk about ‘having someone there for me, someone to talk things through with, someone who has some solidity’. Rarely will people talk about having a hot love. It’s about companionship.”

Millie Thompson, a 29-year-old marketing executive, is typical. Next month she will marry her partner of eight years in a £10,000 ceremony near her parents’ home in Cheshire. A picturesque barn will be hired for the day, the registrar will marry them there, and a meal and party for 80 guests will follow. Thompson was privately educated and acknowledges that she has been influenced by her parents’ enduring marriage – and their expectations.

“There’s a pragmatic part of me that says we’ve bought a house together – what if anything happens? Our legal status isn’t protected as far as I’m aware. And there’s marriage as an expression of love in front of the people who are important to me. This is the person who I want to be with for the rest of my life. It’s reassuring and stabilising to say that and mean it. I’m not religious, but the venue has a certain gravity and feels solemn, which is important to us. I like the notion of throwing a big party and saying to everyone, ‘Come and enjoy yourself’. Starting a family is at the back of my mind as well.”

Declining numbers of church weddings have led the Church of England to examine how it can entice more couples through its doors. Its recent report on the subject confirms that there are still plenty of people who want to marry – and that the Church should capitalise on this. Some vicars need training to be more punter-friendly, it suggests, and a chatty lady vicar blog has been set up to ram home the point that church weddings need not be stuffy and can be personalised. There has even, heaven help us, been a church wedding roadshow.

So if people are still keen on marriage, why don’t more couples do it? After working on the BBC One series The Big Day, the TV presenter Nick Knowles concludes that it is not the institution that disenchants people, but the perception of a commercialised process, and fears about managing a fractious extended family.

“When people are under financial pressure to get their first foot on the housing ladder and they’re told that they should spend £18,000 on a process that can seem arcane and impersonal, marriage starts to look like a money-making rap and tends to drop to the bottom of your must-do list,” he says. “That’s not because of a lack of romance. People want to involve their families, too, but the divorce rate means that there is often a parent-in-law who won’t be in the same room as someone else. That adds another layer of difficulty.”

Penny Mansfield tells of a couple who decided to build a conservatory instead of having a wedding. It is no coincidence that marriage rates go up when the economy is strong.

PRIME NUMBER

Seven is significant in the Bible, prominent in the Torah and in the Roman Catholic Church’s seven sacraments, seven deadly sins and seven virtues. It is also important within the Hindu and Muslim traditions and the Japanese Shinto religion.

The Big Day starts on BBC One today at 8pm.

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