The summer ferry schedules are usually out by April but are subject to change (remember the off-on-off-on-off never to be seen again Aegli for example?
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If you check on line about a week before travelling you should have a reasonable idea of whether you can catch the 08.30 Dodecanese Seaways or have to take the tourist route via Panormitis.
The Dodecanese Seaways boats lost a lot of credibility after they discovered Halki and suddenly left Symi in the lurch last summer with no boats from Kos on Mondays after years of providing a reliable twice daily service in both directions. This winter they are ignoring Symi completely apart from Sundays. So it is no longer safe to presume that they are a safe bet.