The first same-sex weddings in England and Wales can take place from March 29, 2014, equalities minister Maria Miller has announced.
The date is several months earlier than expected at the time of the passage of the Marriage (Same-Sex Couples) Act in July, when it was thought that gay couples would have to wait until next summer to tie the knot.
Couples wishing to be among the first to marry will need formally to give notice of their intention to marry on March 13.
Ms Miller said: "Marriage is one of our most important institutions, and from March 29, 2014 it will be open to everyone, irrespective of whether they fall in love with someone of the same sex or opposite sex.
"This is just another step in the evolution of marriage and I know that many couples up and down the country will be hugely excited that they can now plan for their big day and demonstrate their love and commitment to each other by getting married."
Ms Miller said she was "working hard" to ensure that couples wishing to convert civil partnerships into marriages - and married people wanting to change their legal gender while remaining married - will be able to do so before the end of next year.
Same-sex couples who married abroad under foreign law and are currently treated as civil partners will be recognised as being married in England and Wales from March 2014.
Those wishing to take part in same-sex weddings in some British consulates and armed forces bases overseas or in military chapels will have to wait until June.
Ben Summerskill, chief executive of gay rights campaign group Stonewall, said he was "delighted" at the announcement, which he said was "particularly resonant" as it came on United Nations Human Rights Day.
"This historic step will mean that, for the first time, every gay person in England and Wales will finally enjoy exactly the same rights as their heterosexual friends and family," said Mr Summerskill.
In a message to same-sex couples making preparations to walk down the aisle together, he added: " Don't forget, if you are thinking of booking a date for your big day in the new year, it's now against the law for any business to treat someone less fairly because of their sexual orientation - whether they're a register office, a hotel or a florist."