They usually run until the end of October - the dates are not always the same each year but it should still be running in mid-October as high season lasts until end of October. What will prevent ferries from getting to and from Positano can be rough seas/weather as it is just a dock, not a harbor. But, I wouldn't worry about it.
At the tail end of the season (i.e. the month of October), the hydrofoil service between Sorrento and Positano MAY not operate daily, and service is always contingent on acceptable weather conditions.
We were in Positano this past October for 10 days and the ferries only ran for 2 of those days due to the sea conditions. if it is the least bit choppy they do not run, so be prepared for other transport options.
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