Not all the guests have their invites and Mr McGraith still does not know what he will be wearing on the day.
But what he does know for certain is how important the marriage will be, not only for his partner and him, but also for gay men and lesbians in the UK and the rest of the world.
"Politically, it's hugely significant to us," says Mr McGraith. "It's our government - and society - recognising gay relationships as equal."
It is also important personally for the 49-year-old Scot. Mr McGraith, who is a campaigner and writer from Lanarkshire, and Mr Cabreza, 42, who works in finance, have been together for 17 years and have two children they adopted six years ago. For Mr Galbraith, the marriage is an important marker in their relationship and a chance for a celebration with about 100 friends and family.
"I'm in love and have a wonderful man and I'm delighted to have him," says Mr Galbraith.
It is also a chance, as far as Mr Galbraith is concerned, to redefine marriage and take it out of the hands of Tories and traditionalists. "I'm not buying into a conventional marriage," he says, "and I don't want people to think that's what this is about. We are changing marriage - it is not marriage changing us."
The ceremony will happen at midnight on Friday, just a few seconds after The Marriage (Same Sex Couples) Act comes into effect. Passed last year, it made gay marriages in England and Wales legal and was followed by Scotland's same-sex marriage bill which was passed last month (the first Scottish gay marriages will happen in the autumn).
The ceremony will take place at Islington Town Hall and will be one of a number of gay marriages taking place in the early hours of Saturday.
Mr McGraith hopes it will be a good day but he also hopes the publicity will act as a positive sign for gay people in countries where they are still persecuted. "Our marriage is happening on a particular day, a day in history, but it's not the end point because as everyone is congratulating us, there are many gay men and lesbians around the world who are not invited to the party and we want to send out a message of hope and solidarity to them.
"If you can't hold hands in the street in India or Iran without fear of violence or imprisonment or worse, that's a serious issue for all of us. I feel part of the same community as those people. Marriage is a political statement and we are making that statement in support of people who can't."
The chief witness at the ceremony will be the couple's friend and gay rights campaigner Peter Tatchell, who said: "Their marriage is a celebration for Peter and David and for the whole lesbian, gay, bisexual and transgender community. It marks the end of the ban on same-sex marriage and is another hugely significant milestone in the quest for lesbian and gay equality.
"The legalisation of same-sex marriage ends the last major legal discrimination against gay people in England and Wales. Scotland will follow later this year. Sadly, Northern Ireland remains a bastion of homophobia."
Mr McGraith does accept that marriage is not for everyone - gay or straight. "I do have my concerns about the institution of marriage," he says. "It can be quite divisive when we hear politicians talking about supporting marriage and I would hate to think that young gay men and lesbians would start becoming David Cameron's new sanitised gays - I don't want anyone to think that's what we are.
"So many of the traditions around marriage are also anachronistic and reek of misogyny so there'll be nobody being 'given away' on Friday and no virginal white."
David and Peter's wedding will be the modern one: designed to be right for the couple rather than society.