Most Instagram users share their photos instantly. Leila Khan sometimes waits six weeks. That is how long she has held off on posting a photo of her eating a spoonful of watermelon.
Ms. Khan, a 13-year-old from Palo Alto, Calif., makes elaborate preparations for her Instagram posts, and one of her main criteria is finding the right occasion. She finally found a reason to post her watermelon photo: National Watermelon Day, on Monday.
“I’m gonna be like ‘P.S.: It’s National Watermelon Day,’ ” Ms. Khan said to some friends during a recent trip to the mall.
Assuming that National Watermelon Day goes like the many novel national holidays that have grown in step with social media, Ms. Khan will not be alone. People will discuss watermelons on Twitter. Some Grinch might chime in with how many gallons of water it takes drought-stricken California to grow a single watermelon. Maybe others, feeling left out, will ask all their Facebook friends why their feed is suddenly full of watermelons.
Social media did not create National Watermelon Day, which the National Watermelon Association has been celebrating every Aug. 3 for as long as anyone there can remember. Strange holidays are a decades-old tradition that gives trade groups something to promote and newscasters a way to fill airtime.
What social media has done, however, is create the need for billions of people to have something to say. The result is a string of new holidays like Tweed Day andUncommon Instrument Awareness Day that seem to exist only on the Internet.
“Everyone is trying to find something to talk about when there is nothing to talk about,” said John-Bryan Hopkins, a social media consultant who runsFoodimentary, a website dedicated to food holidays.
Ms. Khan does not “have a special thing about watermelons,” and the picture of her eating one was not a momentous event. It was a hot day in June. She and a friend bought a watermelon. Someone took a picture.
Which raised the question: Why wait until Aug. 3 to post the photo?
“It would be weird if I randomly posted a picture of me eating a watermelon,” Ms. Khan explained.
“But if there’s, like, context,” said her friend Lucy Nemerov, also 13.
“National Watermelon Day — then it’s like a post,” Ms. Khan said.
Americans spend a little more than an hour a day using social media, according to the market research company eMarketer. On Instagram and Twitter, every day is a national occasion to post a picture of one’s friends, pets, dinner or haircut.
Whether any of these count as holidays is something of a philosophical question. Take Throwback Thursday — #TBT on social media — a weekly occasion on which millions of people share childhood pictures, outfits with leg warmers and other bygone moments. It is far from a national holiday, yet it is as reliable as Christmas — at least on social media.
“A lot of these things lead into things that become more mainstream,” said Blake Barnes, a product manager at Instagram. “My mom posted a TBT a few weeks ago.”
Cecilia Salas, a 19-year-old college student and prolific user of social media, said she averaged about one “national day” photo a week. Some recent ones includedNational Ice Cream Day, National Mac and Cheese Day and National Hug Your Cat Day.
“Usually I’ll just look at whatever is trending on Twitter, and if I like it, I’ll participate, and if not, I’ll look at the pictures,” she said. “I take a lot of selfies, so I tend to use those if it goes along with the theme.”
On Wednesday, for instance, she posted a photo of the bottom half of her face forNational Lipstick Day. Ms. Salas took the picture three months ago. “I was saving it for the perfect moment,” she said.
Nobody gets the last word on anything on the Internet, and this includes holidays.Chase’s Calendar of Events, which has been published since 1957 and includes hundreds of offbeat holidays, including International Hug a Medievalist Day, is the closest thing to an authority. But today the book has to compete with a growing number of calendar sites like Brownielocks, Checkiday, Wellcat andDays of the Year, which lists about 1,300 holidays, including Milk Chocolate Day,Girlfriends Day and Take Your Houseplant for a Walk Day.
Mr. Hopkins, of Foodimentary, said he had made up somewhere around 120 food holidays. He revises the calendar annually, adding new holidays while retiring others that fail to catch on. This year, Foodimentary will celebrate its first IPA Day, named for the hoppy style of beer. National Noodle Ring Day is out.
Mr. Hopkins, a social media consultant, does not sell advertising on his site. He does it for the attention — and clicks.
But Marlo Anderson has made a business out of holidays, charging clients like theNational Bobblehead Hall of Fame and Museum in Milwaukee $1,500 to have Jan. 7 designated National Bobblehead Day on his site, National Day Calendar.
“I guess we could have declared the day ourselves,” said Phil Sklar, co-founder and chief executive of the bobblehead museum. “But going through the National Day Calendar gives us legitimacy.”
With so many competing calendars, conflicts are unavoidable. Mr. Hopkins, for instance, counts at least four National Brownie Days.
William Wildridge tried to solve that problem by letting the masses decide. His site, WhatNationalDayIsIt.com, uses an algorithm to sort through the millions of Twitter postings containing “national” and “day.” The idea is that today is whatever day Twitter says it is.
He has analyzed 2.3 million Twitter posts and, in the process, received an unfiltered look at the Internet’s collective id. What he has learned is that people say a lot of nasty things on Twitter, but the things that are passed on tend to be heartfelt and positive.
Take the #NationalSexDay hashtag, which gets started a lot but stays low on his list because nobody wants to retweet it. The days that are shared the most are for best friends, siblings, kissing and hugs.
Days of commemoration can be about more than watermelons and hugs, of course. The United Nations has recognized Nelson Mandela’s birthday, July 18, asMandela Day.
But this poses a problem for Jono Alderson, who runs Days of the Year. It is hard to reconcile having a day that recognizes a man who fought apartheid in a calendar that lists days for celebrating lollipops, umbrella covers and cows.
“It’s slightly awkward,” said Mr. Alderson, who lists July 18 as National Caviar Day.
Mr. Alderson’s site is based in York, England, but he says 90 percent of its traffic comes from the United States.
That is not an accident. One of the first things the founders did after revolting against Britain was to ditch the crown’s calendar, says Leigh Eric Schmidt, author of “Consumer Rites: The Buying and Selling of American Holidays.”
Since then, the United States has created a number of new holidays that incorporate two very American traits: self-invention and an excuse to sell products. Trade groups invented most of the lesser-known celebrations, Mr. Schmidt said, but social media has given the power back to the people.
“It seems to me that it’s been taken out of the hands of the trade organizations,” he said. “That’s not where the creativity is coming from anymore.”