Earlier than feels reasonable. Months, not weeks.
The useful part of a roll call — sharing a tour, coordinating anything at all — needs lead time, because people book their port days early. Start a thread the week before you sail and you have made a very quiet thread. Start it six months out and you catch people while they are still deciding, which is when a group actually forms.
Expect silence at first. Almost every roll call sits empty for a while and then fills as the date closes. Post it, check back occasionally, do not take the quiet personally.
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