If you're not saying a traditional "I do" you're joining a growing number of people in the U.S.
"Marriage optional" is one of the top trends that may affect consumer behavior and mind set in 2012, according to experts at JWT, a marketing communications company based in New York City.
In 2000, more than 3 million unmarried couples cohabited; in 2011 that number more than doubled to 7.6 million couples, according to the U.S. Census Bureau.
When family and friends learn of your event, they may have expectations that don't match yours.
Be very clear about what you're celebrating so your guests aren't confused or disappointed, say party experts.
Your invitation can be formal or casual, but it should avoid using the word wedding - even to explain what you're not doing.
Instead, use "something as simple as 'Come celebrate with us for our X years together.' Something fun and cute," says Sharon Ringier of Sharon Ringier Events in the Chicago area.
Whether your invitation extends to the guest's companion depends on your style and budget.
If you enjoy big parties and aren't worried about expenses, your invitation can include a plus-one, says Marley Majcher.
Add the plus-one if the recipient is married, engaged or living with someone, even if your partner doesn't know the couple, Majcher advises.
Gift giving is another element you should address in the invitation. Even without the wedding aspect, your guests will wonder whether gifts are acceptable.
They're not, according to Erin Grace.
"In these situations couples have been together. They may have lived together; they don't need to ask for gifts," says Grace, who runs her own event-planning firm in Los Angeles.
However, if you really want to discourage gift giving, say so in your invitation.
"If you don't want gifts, put no gifts please. It's appropriate here, where it's not for weddings," says Anna Post, great-great-granddaughter of Emily Post and co-author of "Emily Post's Etiquette, 18th Edition" (William Morrow, 2011).
As an alternative, add a card suggesting a donation to a specific charity instead of a present, says Ringier.
Whether you're planning your celebration yourselves or through an event planner, you have details to manage.
You won't have bridesmaids or a best man to assist, but you can ask those near and dear to pitch in. Friends brewed two kegs of beer and supplied glassware for Post's housewarming party with her boyfriend.
"I was touched by how helpful everyone was. Our friends were looking for things to do to help," says Post, who suggests a mutual round of toasts that acknowledges everyone. "The couple can thank their guests, then offer a toast to each other. Then maybe close friends can offer toasts as well," Post says.