The first same-sex weddings can take place from 29 March 2014, Equalities Minister Maria Miller says.
Initially it was thought the first same-sex marriage in England and Wales would not take place until the summer.
Couples wishing to be among the first to marry will need to give formal notice of their intention to marry on 13 March.
Ms Miller said: "Marriage is one of our most important institutions, and from 29 March 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 added that she was "working hard" to ensure that couples who wanted to convert civil partnerships into marriages - and married people wanting to change their legal gender while remaining married - would be able to do so before the end of next year.
From June, people will also be able to take part in same-sex weddings in some British consulates and armed forces bases overseas or in military chapels.
