National Bikini Day is a national holiday that fell on Wednesday.
And stars worked hard to compete on Instagram for the big day, starting early in the morning and finishing after midnight.
Most posed in bikinis which made the most of their fabulous figures, and a few sported monokinis, while others played it safer by wearing a one piece.
But all of the ladies looked sensational as they were toned and tanned.
Frida actress Salma Hayek put on an eye-popping display with a multicolored patchwork two piece while makeup-free in a swimming pool. She said the image was ‘NOT a throwback.’
Cosmetic mogul Kylie Jenner shared a flashback photo where she had on a bikini with heels while in Mexico – she added a skirt and sheer top.
Her sister Kim Kardashian looked slender in a light blue bikini from SKIMS while on a white sand beach with her long black hair down.
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Bikini time: National Bikini Day is a national holiday that fell on Wednesday. And stars worked hard to compete on Instagram for the big event. Salma Hayek put on an eye-popping display Vote below
Kimmy: Her sister Kim Kardashian looked slender in a light blue bikini from SKIMS while on a white sand beach with her long black hair down
Selling Sunset star Emma Hernan was in a hot pink two piece as she took a mirror selfie in Palm Springs. She looked like a Victoria’s Secret model.
Model Lori Harvey – who is the daughter of comedian Steve Harvey – sizzled in a black bikini set from PrettyLittleThing as she posed outdoors.
Vogue favorite Hailey Bieber was in a red bikini top as she held up a cherry and sported a cap – her husband Justin Bieber was not seen.
Kanye West’s former girlfriend Julia Fox showed off her lean frame in a two piece as she wore heels by a pool and held onto a purse.
The Hills vet Kristin Cavallari modeled a yellow two piece while on vacation with her male friends. The mother of three looked slim and had a glow.
Racy: Kylie Jenner shared a flashback photo where she had on a bikini with heels while in Mexico
Hot as can be: Selling Sunset star Emma Hernan was in a hot pink two piece as she took a mirror selfie in Palm Springs.
Melissa Gorga of Real Housewives of New Jersey wore a black strappy wonder while on the beach on the East Coast. She added a shirt that was worn open.
Brittany Cartwright of Vanderpump Rules wore a one-piece Fendi suit on a boat during a girls trip.
Coco Austin had on a white tiny thong and showed underboob with a red Arizona crop top; her husband Ice-T defended her against trolls who didn’t like the look.
Luann de Lesseps of RHONY fame had on a royal blue bikini with gold accents while on a speedboat with a pal.
Olivia Culpo of The Culpo Sisters showed off her bottom in a brown bikini while in North Carolina.
Heather Rae El Moussa wore a hot pink bikini as she was hugged by her husband Tarek.
Fire: Model Lori Harvey sizzled in a black bikini set from PrettyLittleThing as she posted outdoors
Haute chick: Vogue favorite Hailey Bieber was in a red bikini top as she held up a cherry and sported a cap
Kanye’s ex: West’s former girlfriend Julia Fox showed off her lean frame in a two piece as she wore heels by a pool and held onto a purse
Hills is behind her: The Hills vet Kristin Cavallari modeled a yellow two piece while on vacation with her male friends
She also posed in the mirror to show off this look
Waves time: Melissa Gorga of Real Housewives of New Jersey wore a black strappy wonder while on the beach on the East Coast. She added a shirt that was worn open
Lulu’s look: Luann de Lesseps had on a royal blue bikini with gold accents while on a speedboat with a pal
So much to take in: Coco Austin had on a white tiny thong and showed underboob with a red Arizona crop top; her husband Ice-T defended her against trolls who didn’t like the look
Miss World does it again: Olivia Culpo of The Culpo Sisters showed off her bottom in a brown bikini while in North Carolina
Rae of light: Heather Rae El Moussa wore a hot pink bikini as she was hugged by her husband Tarek
She is with child: Jana Kramer showed off her baby bump in a yellow string bikini as she hugged one of her kids
Polka person: Ashley Greene wore a polka dot bikini as she held onto her baby
Britney being Britney: Britney Spears had on bikini bottoms with white boots
She nailed her look: Lizzo looked cute in her green strappy two piece
Does this count: Jennifer Lopez modeled a one piece swimsuit, which was not a bikini, so she does not get voted on
The bikini has an interesting history.
Ancient Roman women could be seen wearing bikinis when competing in athletic events in wall art.
The Villa Romana del Casale in Italy offers some of these depictions.
Then in the 1930s in the US, bikinis started popping up.
But it was not until material rationing due to War World II that the ladies started wearing much less.
The bikini has often been dated to July 5, 1946.
That is when French engineer Louis Réard introduced the modern bikini.
The sexy design was modeled by Micheline Bernardini, a French former nude dancer. The suit was a string bikini with a high waist and triangle top. It did not look too unlike what women wear today.
Reard named his design for the Bikini Atoll, where the first post-war tests of the atomic bomb were taking place.
French women welcomed the design but others thought it was much too scandalous as it flashed plenty of flesh.
In 1953 Brigitte Bardot made bikini popular when she was seen in one at the Cannes Film Festival.
Next Hollywood bombshells Rita Hayworth and Ava Gardner slipped into two pieces for media attention.
In the 1960s bikinis took off thanks to movies.
Ursula Andress was unforgettable as she emerged from the surf of a pristine beach wearing a two piece in the James Bond movie Dr No in 1962.
Raquel Welch had on a fur bikini for the film One Million Years B.C. in 1966.
Today many celebrities have their own bikini lines.
Kim Kardashian owns SKIMS SWIM.
Devon Windsor has her own line that includes jackets.
Candice Swanepoel has done well with her brand Tropic Of C.
Pamela Anderson has collaborated with Frankies Bikinis and Brittany Cartwright with Cupshe.
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