"Invasion!"
 is the third annual Arrowverse crossover event, crossing over episodes of the Arrowverse television series "The Flash", "Arrow" and "Legends of Tomorrow" on The CW, and is inspired by the 1989 comic miniseries "Invasion!
".
 Events at the end of the "Supergirl" episode "Medusa" on November 28, 2016 marked the beginning of the crossover, which officially began on November 29 with "The Flash", continued on "Arrow" on November 30, and concluded on "Legends of Tomorrow" on December 1.
 Each of the latter three episodes were titled "Invasion!"
.
 "Invasion!"
 sees Barry Allen recruit Kara Danvers / Supergirl to his Earth to help his team, Oliver Queen and his team, and the Legends to defeat the alien race known as the Dominators.
"Heroes Join Forces" is the second annual Arrowverse crossover event, featuring episodes of the television series "The Flash" and "Arrow" on The CW.
 The event began on December 1, 2015, with "The Flash" episode "Legends of Today" and concluded the following day with the "Arrow" episode "Legends of Yesterday".
 The episodes feature characters from "DC's Legends of Tomorrow", which was being developed as a spin-off.
 The crossover sees Barry Allen / Flash and Oliver Queen / Arrow team up to take on Vandal Savage, who is looking for Kendra Saunders and Carter Hall, the reincarnations of Hawkgirl and Hawkman.
"Valley Girls" is the twenty-fourth episode of the second season of The CW television series "Gossip Girl".
 The episode served as a backdoor pilot for a potential "Gossip Girl" spin-off series set in the 1980s, entitled "Valley Girls".
 The episode was directed by Mark Piznarski and written by Josh Schwartz and Stephanie Savage (this was the first episode of the series since "Much 'I Do' About Nothing" to be co-written by Schwartz).
 It was filmed on location in New York City, New York and in Los Angeles, California.
 References to elements of 1980s popular culture were heavily accentuated in the episode, which the producers hoped would bridge the generation gap between the characters and audience.
 "Valley Girls" aired on the CW in the United States on May 11, 2009 and was viewed live by an audience of 2.31 million Americans.
 Although the episode received generally positive reviews, the spin-off series was not picked up.
"162 Candles" is the eighth episode of the first season of The CW television series, "The Vampire Diaries" and the eighth episode of the series overall.
 It originally aired on November 5, 2009.
 The episode was written by Barbie Kligman and Gabrielle Stanton and directed by Rick Bota.
"Duet" is the seventeenth episode of the third season of The CW television series "The Flash", which aired on March 21, 2017.
 The episode was written by Aaron Helbing and Todd Helbing from a story by Greg Berlanti and Andrew Kreisberg, and directed by Dermott Daniel Downs.
 The episode features a musical crossover with "Supergirl" and reunites Grant Gustin and Melissa Benoist with their former "Glee" co-star Darren Criss, who portrays the Music Meister.
 Jesse L. Martin also reunites with Jeremy Jordan, whom he previously worked with in the 2012 musical film "Joyful Noise".
 The story begins at the end of the "Supergirl" episode "Star-Crossed" on March 20, 2017, which sees Kara Danvers / Supergirl (Benoist) being put in a coma by Music Meister, with Mon-El (Chris Wood) and J'onn J'onzz / Martian Manhunter (David Harewood) bringing her to Earth-1 in hope that Team Flash can save her.
"The Adventures of Supergirl" is the first episode and premiere of the second season from The CW television series "Supergirl", which aired on October 10, 2016.
 It is the first new episode of "Supergirl" to air on the CW since moving from CBS, and the first to feature a prominent appearance by Superman, portrayed by Tyler Hoechlin.
"Flash vs. Arrow" is the first annual Arrowverse crossover event, broadcast on The CW, featuring episodes of the Arrowverse television series "The Flash" and "Arrow".
 The event began on December 2, 2014, with "The Flash" episode "Flash vs. Arrow" and concluded the next day with the "Arrow" episode "The Brave and The Bold".
 The crossover sees Team Flash (Barry Allen / Flash, Caitlin Snow, and Cisco Ramon) helping Team Arrow (Oliver Queen / Arrow, Felicity Smoak, and John Diggle) take on the boomerang-wielding villain Captain Boomerang, while Team Arrow helps Team Flash confront the metahuman Roy Bivolo.
"Crisis on Earth-X" is the fourth annual Arrowverse crossover event, crossing over episodes of the live-action television series "Supergirl", "Arrow", "The Flash", and "Legends of Tomorrow" on The CW.
 The crossover will begin on November 27, 2017 with "Supergirl" and "Arrow", and conclude on November 28 with "The Flash" and "Legends of Tomorrow".
 "Crisis on Earth-X" sees Barry Allen and Iris West's friends come to Central City for their wedding, only to be interrupted when villains from Earth-X disrupt the proceedings.
"Supergirl Lives" is the 9th episode of the second season from The CW television series "Supergirl", which aired on January 23, 2017.
 The episode was directed by Kevin Smith and was based on a planned movie that he wanted to direct, "Superman Lives," which was scrapped afterwards.
 The episode is also a family affair, as his daughter Harley Quinn Smith appears in this outing, with Kevin appearing in a cameo.
"Medusa" is the eighth episode of the second season from The CW television series "Supergirl", which aired on November 21, 2016.
 The episode features a minor tie-in with the Arrowverse crossover event "Invasion!
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Question: "Medusa" is the eighth episode of the second season from The CW television series "Supergirl", which aired on which date, the episode features a minor tie-in with the Arrowverse crossover event "Invasion!"?
Answer: November 21, 2016