As with Invasion from last season, you need to watch the corresponding episodes in Supergirl, Arrow and Legends of Tomorrow for this to make sense. While I saw a bunch of reviews by people frustrated with having to watch other shows, this is complaining on a very high level. With streaming at your fingertips, you don't need to watch six seasons of Arrow, instead you can just jump to the connected episode and it all makes sense. They do a nice way of explaining plot points, relationships and characters, that you would know, if you watched all the other shows, but it makes sense without it. I only watch Flash (though I did watch half of the first season of Arrow) but if you made it through three and a half seasons of Flash, you already know all the characters from previous crossovers. It's not overly complicated. I actually love crossovers because they logically connect characters in the Arrowverse that should know each other. I find it actually more confusing, if you watch other shows that have a connection and characters never meet when they visit the respective cities. It's also in line with the comic book universe these shows were born in, where the connection of the characters and the multiverse is well established. When you watch the show and pay attention to the themes and philosophy it uses, it makes most sense to put the West-Allen wedding into such a crisis episode because love and family gives the hero the whole reason and purpose to fight in the first place.
I would prefer though, if there was a disclaimer before this episode in Flash to tell you which episodes to watch, so I wouldn't have to google it. So here is the info for you:
This is the third part of a four-part crossover event, it begun in Supergirl episode 3x8 "Crisis on Earth-X, Part 1," then Arrow episode 6x8 "Crisis on Earth-X, Part 2," and continued after this episode and concluded in DCs Legends of Tomorrow episode 3x8 "Crisis on Earth-X, Part 4."
I would prefer though, if there was a disclaimer before this episode in Flash to tell you which episodes to watch, so I wouldn't have to google it. So here is the info for you:
This is the third part of a four-part crossover event, it begun in Supergirl episode 3x8 "Crisis on Earth-X, Part 1," then Arrow episode 6x8 "Crisis on Earth-X, Part 2," and continued after this episode and concluded in DCs Legends of Tomorrow episode 3x8 "Crisis on Earth-X, Part 4."