If you’re into the doujinshi world, even since only a few months, I’m pretty sure you have come across this event.
The Comic Market (shortened as “Comiket xx” or “Cxx”, where “xx” is Comiket’s number) is the biggest doujinshi event held at Tokyo Big Sight, in Tokyo, twice a year (August and December) and is divided in three days. During it, one can find music CDs, anime, games, manga, figures and so on. If you can imagine something, you will most likely be able to find it during Comiket.
The first day is dedicated to doujinshi based on original works (such as Type-Moon’s games, Kadokawa’s mangas, Girls und Panzer and Kancolle). The second day is dedicated to derivative works from other doujinshi (such as Higurashi and Touhou). The third and final day is dedicated to original doujinshi.
Comic Market is also known to be one of biggest event for cosplayers, with dedicated cosplay areas where you can photo shooting –and trust me, they are very strict when it comes time to take a picture and you’re not in a cosplay area.
As of today, December the 10th, 2016, the most recent Comic Market was held between the 12th and the 14th of August, 2016 (C90), while the next one will be held between the 29th and the 31st of December , 2016 (C91).
 Note that this isn’t an exhaustive list, nor established or confirmed anywhere. It is based on my experience and what I have seen during Comiket. As for the examples, those mentioned are the works I remember to have seen most derivative works off as of Comiket 90.