Now, on alternate Saturdays, the steamers of the Castle line sail from Southampton, where they call to take up passengers and mails.

Was this one of those alternate Saturdays?
I looked at the list of dates: it was.

That told further in favour of Southampton.

But did any steamer of any passenger line sail from Plymouth on the same day?
None, that I could find.

Or from Southampton elsewhere?
I looked them all up.