The First Slap
In the season 2 episode "Slap Bet", the gang tries to find out why Robin is so against going to the mall. Marshall and Barney have a bet riding on whether she got married in a mall or did porn and they ultimately agree to a slap bet where whoever wins gets to slap the other as hard as they can. Ultimately both guys were wrong in their guesses, and although both slapped one another pre-maturely, Marshall gives Barney the option to receive 10 slaps immediately or receive 5 slaps over a period of time. Barney chooses the five slaps and thus it all begins..
The Second Slap
In the season 2 episode "Stuff", Barney seeks revenge on Lily after she made the gang go see her play which he confessed was awful. Barney decides to put on a one man show as payback and Lily ultimately admits she has nothing nice to say about his show and the group is ready to leave. Barney begins to tap dance leading into the finale of his show, but Marshall gets up and uses his second slap to make Barney stop.
The Third Slap
In the perfectly titled "Slapsgiving" episode, Marshall creates a countdown to his third slap for Barney. Although Lily originally stated no slap would occur on Thanksgiving, Barney taunts Marshall as the countdown gets to its final 10 seconds. Despite Lily's warnings, Barney continues his taunts and Lily ultimately allows Marshall to slap Barney with 3 seconds left on the countdown and it was slap-mazing.
The Fourth Slap
In "Slapsgiving 2: Revenge of the Slap", Marshall allows Ted and Robin to have the pleasure of slapping Barney. Ted ultimately gifts the slap to Robin who then gives it to Mickey as a welcome to the group. Mickey decides to gift the slap to Lily as an olive branch to make up for his absence as a father. Lily ultimately can't bring herself to slap Barney and Marshall explains he gave the slap to bring everyone together. Barney's released from the chair he was tied to and... then Marshall gets his sweet revenge.
The Fifth and Sixth Slap
In "Disaster Averted" Barney gets tired of wearing the ducky tie and doesn't want to wear it the rest of the year. Marshall and Lily agree he can take it off, but in return Marshall gets 3 slaps added to his final existing one. Marshall wastes no time in slapping Barney TWICE this episode.
The Seventh Slap
In "Slapsgiving 3: Slappappointment in Slappmarra", Marshall explains how he mastered the Slap of a Million Exploding Suns in order to deliver the epic penultimate slap to Barney 17 hours before his wedding. Boyz II Men come out of nowhere to sing You Just Got Slapped and it was all kinds of slap-tacular.
The Final Slap
In "The End of the Aisle", the last episode before the series finale, Marshall delivers the final slap to Barney right as he is in the middle of a panic before Robin walks down the aisle. How fitting for the final slap to be Marshall slapping the sense back into Barney. To say the timing was perfect doesn't even begin to cover it, but it was the most LEGEN…WAIT FOR IT...DARY way to end the slap bet at the altar.
From finding the final doppelgänger to watching Marshall slap Barney for the last time, How I Met Your Mother is finally wrapping up. Thank you for all of the memories, the laughs, the tears and the slaps. Only one more episode left..