Ogechukwu Anwaegbu invited her friends on a three-day getaway to New Orleans last year to celebrate turning 24 and a half. They thought she was half joking.

“There was lots of side-eye,” said the Houston medical-school student, now 25.

Her friends went along with it anyway, accompanying her to museums and bars, and a two-hour boat party on Lake Pontchartrain. She paid $1,000 for the trip and her friends paid about the same.

Half birthdays are the latest milestone, sort of, for adults. They are borrowing an idea more often associated with toddlers, and instead throwing themed picnics, booking DJs and planning vacations that run into the thousands of dollars.

For younger adults especially, traditional milestones like weddings are happening later in life . That has left a void in the calendar that made-up milestones are helping to fill, behavioral scientists say.

“It’s not just a random day; it’s now someone’s half birthday so you feel like you must step up and make it happen,” said Michael Norton, a professor at Harvard Business School who has studied habits around spending and rituals.

American Greetings, which offers half-birthday cards, says its research shows that 63% of consumers are celebrating less conventional occasions, such as gender-reveal parties, Galentine’s Day and half birthdays.

Searches on Etsy for half-birthday cards, gifts and decorations jumped 57% in the first three months of 2024 compared with a year earlier, the online marketplace said.

Cheyanne Carroll, 30, a graphic designer who runs an Etsy shop out of Orlando, Fla., began selling half-birthday cards in January. Her card, which cuts the words “Happy Half Birthday!” in half, outsells most others in her store by two to three times, she said.

Revelers are posting on social media about the best half-birthday gifts (half of a real birthday gift, of course) and writing blogs about the best ways to celebrate.

Grocery stores and restaurant chains including Publix, Chipotle and Baskin Robbins now email discounts and freebies to customers on their half birthdays.

At Valley & Co. Events in Seattle, demand for unconventional milestone events was up more than 30% in 2023 versus 2022, said Aleah Valley, the company’s co-founder. Besides half birthdays, there were also parties marking the halfway point of home renovations, she said.

The birthday bill

When it comes to adult birthdays, the ones without a zero at the end tend to be marginal affairs. Still, birthday parties are the most common type of celebration Americans attend, and one of the most enjoyed, a 2023 poll by YouGov found.

They are also among the most expensive. Many of the forces driving up the costs of wedding attendance and teen parties have an impact on unconventional celebrations. The rapid increase in prices for travel and dining out— or “funflation ”—has hit birthday budgets, too.

Birthday celebrations are a top reason millennials overspend in their friendships, according to a Credit Karma survey .

Party inflation is partly driven by the increasing budgets of the wealthy and the efforts of others to match them, said Robert Frank, an economist at Cornell University who studies how social environments shape consumer spending.

“It cascades all the way down,” Frank said.

Talia Foster, 45, an entrepreneur based in Detroit, and her daughter treated themselves to a luxury getaway last year in Cancún to celebrate their half birthdays.

The six-day vacation at an all-inclusive resort, came to about $6,000. They went on excursions to swim with dolphins and ziplined.

They also commemorated the occasion with a professional beach photo shoot, in flowing gowns. This midyear celebration allowed them to accommodate her daughter’s new job schedule, providing her with ample time to settle in before requesting time off for their retreat.

“We’re the type of family to celebrate any, and everything,” Foster said. “For your birthday, you can have it how you want it, even if that means changing the date.”

When Remaz Abdelrhman received an invitation to a friend’s half-birthday picnic last June, she looked past her initial skepticism and decided to help plan it. The 24-year-old brought an orange color-themed basket filled with picnic items, and wore an orange outfit to match. She enjoyed reconnecting with friends she seldom sees, she said. It cost her under $40.

Magis Musa, 27, who invited Abdelrhman, had hoped for gifts, but she didn’t want to be presumptuous. Her friends didn’t bring any.

“You can’t really demand them,” she said. Still, the party was successful enough that she plans to go bigger this year for her half birthday.

Don’t overextend yourself

Americans believe it is appropriate to spend about $77 on birthday gifts for significant others and $54 for close friends, according to a 2023 survey by the gifting website Snappy.

But etiquette coaches agree that gifts aren’t required for half birthdays.

“You don’t have to overextend yourself for a made-up event,” said Elaine Swann, founder of the Carlsbad, Calif.-based Swann School of Protocol. “It usually doesn’t carry the same weight.”

Last June, Amalia Karaman, 22, opted for a no-gifts-allowed policy during her half-birthday party at her parents’ home in Nashville, Tenn., which included a DJ and 20 friends. Still, to mark the occasion in a memorable way, a friend brought a birthday cake decorated with “20.5” on top.

Some find creative ways to make their celebrations more affordable.

Shaka Smith, a 36-year-old lawyer in Los Angeles, said that when planning his half birthday, he often tries to arrange discounts with bars and restaurants in return for promoting them on social media. When that doesn’t work, friends usually cover the cost of drinks and food, he said.

His actual birthday is a time for introspection, he said, so he celebrates his half birthday with nightclub outings, brunch with friends and vacations.

“No one is expected to say ‘happy birthday’ to me,” Smith said. “Showing up for my half birthday is enough to show you are a good friend.”

Write to Dalvin Brown at dalvin.brown@wsj.com