The reason why they are celebrating is because this artwork is meant as a 'thank you' for playing the mobile game until now (the 'name' of the card is '無ﾊｯﾋﾟｰｴﾝﾄﾞのお祝い(UR)' = 'Celebration of the non-happy ending (UR)', not sure what they mean by that), the game is ending soon, in december, I don't know why they decided to do that one month prior though, can we assume this is the last illustration Clamp will draw? Who knows...EDIT: as written bellow, after the pic, they will stop releasing any new material starting Oct 31th at 15h, meaning the likeliness of this being the last artwork is very high.
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EDIT (Nov1st): full view available! And behold: first drawing of Eriol in 13 years!
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EDIT: from the look of the following text published, it seems like there will be no new elements added to the game from now on, meaning this might indeed be the very last CCS artwork ;_; I just looked again at Clamp's schedule and the nothingness in it really made me sigh...ugh D: The next few months will be very boring...
IMPORTANT: when new pictures/views or versions in better quality are available of all the new artworks for the mobile game, the only thread I update with the new material is this one: CCS mobile game: the new illustrations [thread updated when new arts are available] since having to hunt for/update separately each thread with each different artwork would be bothersome.