It's a day that arrives once in a century: 11/11/11. The unique pattern of numbers has inspired weddings, New Age ceremonies, and even cesarean births.
Our fascination with this and other numerical patterns is no mystery, according to one anthropology professor. It's a by-product of human evolution.
"If you're walking down a rainforest path and you see something that could either be a root or a snake, it's better to assume it's a snake and step around it carefully than to think it's a root," said John Hoopes, an associate professor of anthropology at the University of Kansas.
Our senses are "geared toward pattern recognition," Hoopes told ABCNews.com, "because it gives us an advantage for survival."
"If you're going to pick a number to focus on, '1' is a good place to start," he said. "All these ones are something people notice and pay attention to."
Getting Married on 11/11/11
Katie Barron, 26, who is marrying Victor Thatcher, 31, today at the Bellagio in Las Vegas, told ABCNews.com she and her boyfriend booked the wedding date two years ago.
"We've always known it to be lucky," she said. "If you're somewhere where the clock says 11:11:11 you're supposed to make a wish. We were just brought up on it, I guess."
Brides magazine estimates more than 46,000 weddings will take place today, which is 10 times more than a typical Friday in November.
Jackie Lebowitz, assistant managing editor of the magazine, told ABCNews.com it's "a really lucky number for some couples."
Brides adhere to a lot of superstitions, she said -- such as the old adages that suggest wearing something borrowed or blue. And then there's the dictum that you shouldn't let your husband-to-be see you wearing your wedding gown.
It doesn't hurt that 11/11/11, is an especially memorable number for anniversaries.
"With 11, you have the significance of the number one -- two becoming one, the 'number one marriage,' one soul mate, finding 'the one,'" said Lebowitz, not to mention that it's a "once in a lifetime" experience.
Sharon Brown, assistant supervisor for the marriage services division in Clark County, Nev., said the marriage license office staff is working overtime to keep up with all of the applications it's received. On Wednesday, workers processed more than 800 applications, and by mid-day Thursday they were already up to 510.
"Normally, we only have five [staffers] and today we have 10," she said. "Any time there is a numbered day, it's real big in Vegas. July 7, 2007 was our biggest in history."
C-Sections, New Age Ceremonies on 11/11/11
Reuters recently reported expectant mothers in South Korea have "inundated" hospitals requesting that their cesarean section delivery happen today.
The number of appointments for c-section births was 20 percent higher than in previous years because South Koreans want their children to have a Korean resident registration number that begins with "111111."
In the United States, the demand for c-sections doesn't appear to be as great. But one Des Moines obstetrician said he's offering expectant mothers refunds on all his delivery fees if they give birth on the unusual date.
Dr. Ross Valone, who said he is not superstitious, told ABCNews.com he's been practicing for 30 years and thought "it would be fun to do something to give back to people."