econ21 usually opens the Diet Session through a post in the current Diet Thread. It will be easy to spot this time, because the Diet Thread has been locked, so as soon as you see a new one, there you go. As soon as a session is open, Edicts and Charter Amendments can be proposed, seconded and debated. Candidates for Chancellor can also announce themselves and beginning slinging mud at each other.
If you want to get the feel of a Diet Session, the History has links to the beginnings of each one and the voting on legislation. Reviewing those will give you a good feel for what to expect.
edit: Dumb question, but where is the Diet going to be held? Rome is purple.
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