Good question. I am going to say the original publish date (either in Japanese or in English). So, if a volume was published a couple years ago, but was only posted to Mangadex recently, it would not be eligible. If, however, that same title was licensed and had an official English translation come out this year, then it would be eligible again. There can sometimes be years between the different publish dates, and different people access them differently, so I don't want to prevent people that only read officially licensed translations feel like they can't nominate anything or read any of the nominees. Also, I am limiting it to English for my own sake of trying to confirm things on my end when finalizing the nominees.