MCL SIN WEEK is a fandom event dedicated to posting NSFW content on Tumblr; this includes artwork, writing, and other kinds of content about the characters in the game. The first event was created and held December 8-14 2015, started by user (originally known as) the-wickedness-that-is-zai and has been a reoccurring ever since.
To participate, you can share any NSFW content you create on Tumblr while the event is going on. There is no specific date it happens, it is decided at random and can be one or more times per year.
The content MUST be tagged as #mcl sin week, #mcl18, or #nsfw only. MCL Sin Week content should not be tagged this under the main MCL tags to be respectful of those who do not take part in the event and don't wish to see it. It is also advised that non-participants block the tags in order to avoid seeing any unwanted content.