Question: On what holiday is Barney slapped for the third time?
About the article "How I Met Your Mother", section "Season 3".

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Chastened by Marshall's father's fatal heart attack, the gang all go to get their hearts checked, except Barney, who is too scared to go alone.
Lily agrees to accompany him so long as Barney promises her to never lie to his new girlfriend, Nora.
When Barney visits the cardiologist, she requests that he wears a heart monitor for 24 hours, citing a possible arrhythmia.
The next day, Barney and Lily return to the doctor, who notices some oddities in the readout.
She asks Barney what happened at these certain times, and Barney tells the story of the last 24 hours.
When Barney was at dinner with Nora, she began talking about marriage and settling down, causing Barney's heart to race in fear.
Barney eventually says that he wants the same things.
The couple go to MacLaren's, and while Nora is in the restroom, Barney recounts the dinner to his friends and Lily punches him in the chest, leading to a 14-second cardiac arrest.
Barney blackmails the group into lying to Nora in order to play himself up.
Nora returns and tells the gang that her parents are in town and invites Barney to meet them at brunch the next day.
However, Barney later says to Nora that he was lying about wanting to get married and criticized her for telling the other person in a relationship exactly what to expect to happen.
In response, Nora slapped him.
(Lily and the cardiologist both slap Barney during the telling of the events.
)Meanwhile, Robin says she is going to get a dog, but Ted forbids it because he thinks that he will end up caring for it.
Instead, Robin dates a guy nicknamed Scooby, whose behavior mirrors a dog so closely that Ted, Marshall, and Lily make a number of jokes at Robin and Scooby's unwitting expense.
After Robin is called into work, Scooby reveals that he had purchased a big bag of "sandwiches" (the show's euphemism for marijuana).
Ted, Marshall, and Lily make "sandwich" brownies, and while eating them, the three realize that Scooby has disappeared without any identification.
They search for him, but become distracted and stumble across Barney, who points out Scooby urinating on a fire hydrant across the street.
When Scooby turns to run to them and runs straight into traffic, the group watches in horror, which explains another one of Barney's heart oddities.
Barney apologizes for lying to Nora about wanting to settle down, but Lily says that his lie was about lying, and that in truth, Barney actually does want all those things.
When Barney denies this, Lily has the cardiologist check what happened when Barney first saw Nora on their date, and they see that Barney's heart literally skipped a beat.
Lily tells Barney that this is a sign he really wants to settle down.
Barney goes to the brunch to tell Nora about his feelings, but stops at the door, seeing Nora and her family.
He enters the restaurant, making a heartfelt apology.
Nora forgives him, and introduces him to her parents, for whom Barney performs a magic trick.
However, the scene turns out to be a figment of Barney's imagination, and he leaves before Nora sees him.
The episode ends with a flashback of an incident involving Marshall in 2006.
Marshall drops a calzone while walking on the street, but picks it up after ensuring that nobody is looking at him.
As he takes his first bite, however, he sees Barney nearby and slowly walks away in embarrassment. 
1-2-3 Learn   Big World Adventure   A Very Merry Christmas 2012  I Love My Friends  Clean Up, Clean Up  
Barney begins the season with the word, "-dary!"
Robin returns from a trip to Argentina with her new boyfriend, Gael (Enrique Iglesias), and Ted must adjust to life as just her friend, while watching Robin and Gael fawning over each other.
Marshall and Lily decide to move out on their own, falling in love with a place they cannot afford.
Robin learns of Lily's bad credit rating due to her compulsive shopping for designer brands, and forces Lily to tell Marshall.
Despite this, they are able to finally secure their dream apartment, only to discover it's in a bad location and more poorly constructed than they thought (the floor is tilted).
Barney is slapped for the third time on Thanksgiving, which Marshall dubs "Slapsgiving.
"Ted tells his children he met their mother through a story concerning her yellow umbrella.
He finds the umbrella at a club and takes it home after attending a St. Patrick's Day party where his future wife was, although they did not meet.
Ted attempts to woo Stella (Sarah Chalke), a dermatologist he sees to remove an embarrassing butterfly tattoo.
This culminates in a memorable "two-minute date," which incorporates small talk, dinner, a movie, coffee, two cab rides, and a goodnight kiss, all within two minutes.
Robin sleeps with Barney after he comforts her following a break-up with a past Canadian love; Ted is infuriated, and decides to stop being friends with Barney.
Meanwhile, an unknown woman begins to sabotage Barney's attempts to hook up.
His saboteur is revealed to be Abby (Britney Spears), Stella's receptionist, with a vendetta against him for not calling her after they had sex.
In the season finale, Ted gets into a car accident and ends up in hospital, Subsequently Barney gets hit by a bus on the way to visit him and ends up receiving treatment in the same hospital.
Ted realizes Barney really cares about him and they renew their friendship.
It is revealed Barney has true feelings for Robin, while Ted proposes to Stella in an arcade.
Ted, Marshall, Lily, and Barney find out that Robin does not want to go to a mall and won't explain why.
In trying to guess Robin's secret, Ted reveals that he was trapped under a fake boulder at a mall in Ohio when he was nine.
Marshall posits that Robin was married in a mall in Canada, while Barney believes that Robin did pornography.
They agree to a slap bet, where the winner of the bet slaps the loser as hard as he can, and appoint Lily as the slap bet commissioner.
Ted finds Marshall's theory more credible and tries to find out if Robin is married.
After trying to avoid the issue, Robin tells Ted that she got married in a mall, after which her husband moved to Hong Kong without getting a divorce.
Ted promises to keep it a secret, but then tells the gang after Lily begs him.
Marshall slaps Barney, but Barney still scours Canadian pornography for Robin.
The next day, Marshall uses his law library access to search a legal records database.
He tells Ted that there is no record of a marriage license for Robin in Canada.
Ted "lawyers" Robin with a series of questions about her supposed wedding and gets her to admit that she's not really married.
Robin tells Ted that her lie was a test of his trustworthiness and he failed, and so will never know the real secret.
Marshall also tells Lily of his findings, and Lily, as the slap bet commissioner, informs Barney and allows him to slap Marshall three times (one slap because Marshall lied, and two for being prematurely slapped).
Later, Barney gathers them all in the apartment and reveals that a man in Malaysia has found a video with Robin in it, and it will be posted on MySpace.
He refers to her as "Robin Sparkles", to which Robin reacts with humiliation and horror.
Ted tries to stop the video but Barney will not be placated, and Robin allows him to play the video, wishing to be free of the secret.
Barney only plays the opening part of the video, in which Robin (dressed as a schoolgirl) seductively pleads with a teacher not to give her detention.
Barney, supposedly for the sake of Robin's dignity, pauses the video and slaps Marshall.
When Robin asks why, Barney tells her about the bet and that he proved that Robin had done porn earlier in her life.
Robin plays the video, saying it was not porn, though she would have preferred if it was.
It turns out to be a stereotypically '80s bubblegum pop music video where a young Robin (referred to as "Robin Sparkles") sings "Let's Go To the Mall", a minor hit in Canada that made her a teenage pop star who toured malls across the country.
Lily points out that Barney slapped Marshall without having won the bet, and gives Barney a choice for Marshall's retribution: either be slapped ten times immediately in succession, or five times deliverable at any point for eternity.
Barney chooses the five slaps, and must now live in constant fear of being slapped.
Robin tells Ted that she is glad Ted knows her secret now.
They kiss and Marshall slaps Barney, proclaiming, "That's one".
They all continue to enjoy the video, and Marshall menaces Barney. 
"He Who Gets Slapped" is available on DVD via the Warner Archive Collection.