Lauren Parker, 28, and Annabelle Lynn, 22, will lawfully get married at Northcote House, in Sunningdale Park, on Saturday, March 29.
Same-sex marriages received Royal Assent earlier this year and the first ceremonies had been due to be held next summer.
However, last week, Culture Secretary Maria Miller brought forward the date on which the first gay marriages could take place, to March 29.
Miss Parker and Miss Lynn, from Aylesbury, had initially booked the luxury mansion for a civil partnership ceremony, but will formally marry following the legislation change.
Their marriage will be the second-ever legal same-sex marriage in England and Wales, following a ceremony in a registry office in London earlier the same day.
The couple, who have been together three years and engaged for a year-and-a-half, count themselves lucky to be one of the first gay couples to marry after originally booking the ceremony for the week before. Miss Parker said: “We are both really, really happy and it will be a very special day. We count ourselves very lucky that we will be marrying on the first day of the new law.
“My mum is ecstatic and everyone is very, very happy for us. It has taken a long time for everyone to get their heads around the relationship but personally, my parents are over the moon.
“We thought it was never going to happen and we are quite glad because we did not want to wait any longer. We are pretty excited.”
In the light of the new legislation, Miss Parker hopes attitudes in society will also change in line with the updated law.
“We are both for equality so hopefully now people will see same-sex partnerships in a different light. I’m hoping it will change attitudes. Sometimes it is a bit hard to go out as a couple because it’s frowned upon in most places.”