Warning: SPOILERS for Cobra Kai Season 3
Cobra Kai season 3's finale is titled "December 19" for a good reason because it's a pivotal date in The Karate Kid's legacy: December 19, 1984, is the date of the All Valley Under 18 Karate Tournament when Daniel LaRusso (Ralph Macchio) defeated Johnny Lawrence (William Zabka). Cobra Kai ingeniously reframed this date in season 3 to become another crucial day in the history of the saga.
In The Karate Kid timeline, December 19, 1984, was one day after Daniel LaRusso's birthday and it was when his score with Johnny and the Cobra Kai was originally settled (at the time). As per the deal Mr. Miyagi (Noriyuki "Pat" Morita) struck with John Kreese (Martin Kove) after Halloween, Daniel was afforded the interim weeks to train so that he could face the Cobra Kai in the All Valley Karate Tournament. (The date of the tourney is clearly visible on the poster hanging on the Cobra Kai dojo's wall.) After Daniel won, Kreese then attacked Johnny in the sports arena parking lot, which led to Miyagi humiliating Cobra Kai's sensei. Johnny and the rest of Kreese's students abandoned him and Cobra Kai was shut down until Kreese and Terry Silver (Thomas Ian Griffith) attempted to resurrect the dojo a year later in The Karate Kid Part III - a scheme that LaRusso foiled again by defeating their handpicked Cobra Kai, Mike Barnes (Sean Kanan), to win his second All Valley Championship.
Cobra Kai season 3 takes place in late 2018, beginning two weeks after the West Valley High brawl that occurred on the first day of school and climaxing with another brawl at the LaRussos' home on December 19, 2018. The significant date actually spans the final two episodes of Cobra Kai season 3, "Feel the Night" and "December 19". On the morning of December 19, 2018, Johnny woke up in bed with Carmen Diaz (Vanessa Rubio), the mother of his student and closest friend, Miguel (Xolo Mariduena). Johnny then went on a lunch date with Ali Mills Schwarber (Elisabeth Shue), his ex-girlfriend who was back in LA for the holidays, and it was the first time they saw each other since the events of The Karate Kid back in 1984. Lunch went so well that Ali and Johnny decided to continue their date at Golf N' Stuff before she invited him to a Christmas party at the Encino Oaks Country Club.
Another major event that occurred on December 19th was Miguel visiting the Miyagi-Do dojo at Samantha LaRusso's (Mary Mouser) invitation, where he began to learn about Daniel's late sensei. Daniel mistakenly walked in on Sam and Miguel kissing, but he sat down to get to know Miguel and they finally were able to connect. LaRusso told Diaz his side of The Karate Kid story (Johnny told Miguel his version in Cobra Kai season 1), and they parted on respectful terms as two former All Valley Champions. Later that evening, Daniel and his wife Amanda (Courtney Henggeler) ran into Ali and Johnny at the country club party and they all reminisced about Daniel, Johnny, and Ali's senior year of high school - and how both guys blew it with Ali. Thanks to Ali, Daniel and Johnny left the party as friends.
Meanwhile, Sam and Miguel brought the students of Miyagi-Do and Eagle Fang Karate together at the LaRussos' home and they all agreed to combine their forces. After Cobra Kai members Hawk (Jacob Bertrand), Tory Nichols (Peyton List), Kyler (Joe Seo), and Robby Keene (Tanner Buchanan) stole a snake from the Ventura Zoo, Kreese told them that Miyagi-D0 and Eagle Fang were teaming up against them. This provoked Tory to lead Cobra Kai in an invasion of the LaRussos' home, and their brawl trashed the place. However, Hawk had a change of heart and betrayed Cobra Kai, which turned the fight in favor of Miyagi-Do and Eagle Fang. Tory retreated from her personal brawl with Sam when she realized that Cobra Kai lost the fight.
"December 19" ended with a violent confrontation between Johnny, Kreese, and Daniel at the Cobra Kai dojo after Lawrence and LaRusso learned of the fight at Daniel's house. Daniel might even have killed Kreese with Miyagi's secret he learned in Okinawa if Miguel and Sam didn't stop him. Ultimately, Kreese, who now had Robby firmly on his side, agreed to settle all their differences at the next All Valley Karate Tournament in May 2019, which will happen in Cobra Kai season 4. But the tumultuous events of "December 19" will be long remembered by everyone in Cobra Kai because, just like in 1984, Johnny and Daniel settled their differences once again but Cobra Kai remains a problem that needs to be solved.