Well...right now the ferries are on Tue - Thu - Sun. But who knows what they will be in October! That's a different season and schedules may or may not change. There are no guarantees.
I'm sorry, I wish there was a better answer.
Here's a couple more options, assuming you get to Athens and have to wait 2-3 days:
1. See if there's a ferry to Kos. We've done this once when we accidentally went to the port a day early :) There's a 'Blue Star' ferry to Kos almost every day departing from the same gate. When you get to Kos Town you can either a) take the next Catamaran to Kalymnos IF you arrive at Kos in the morning [the catamaran connects Kos Town to Kalymnos once a day late morning], or b) go to Mastichari port by taxi or bus and take the next ferry to Kalymnos [local ferries connect Mastichari port to Kalymnos several times throughout the day]. Neither option is as cheap as ferrying directly to Kalymnos, but it's probably still cheaper than airfare.
2. If you end up stuck in Athens and don't feel like going to the acropolis, email us and we'll tell you where you can climb. Believe it or not there are quite a few crags on excellent rock all around the city.