Cruise from Stockholm to Helsinki

No dress code as far as I know. But don't dress too casual if you don't want to stand out.

Call Viking and ask them directly.

Alternatively watch YouTube videos of Viking cruises. That timeframe in particular. Also, Cruise Critic forum (sister to TripAdvisor but all about cruising) will offer lots of recommendations on the Viking boards.

After an incident like this -- https://community.ricksteves.com/travel-forum/norway/unfortunate-outbreak-among-ship-s-crew -- my somewhat tongue in cheek answer about a dress, code is "Operating Room" or "EVA space suit".

Hopefully things will be much better by Nov.

Make sure to read about the latest outbreak of virus on European cruise ships.

I don't think Cruise Critic will be very helpful in this case, but you could always try.

What country's passport will you pack, along with your evening attire? Not to sound snarky but seriously, if you are from the US, not many countries currently (and understandably) "want us" visiting or even passing through. Unless the dire situation in the US radically changes by October, what to wear would be at the bottom of my worry list. I had to cancel my late- August small ship cruise to Greenland's west coast and really wonder if it will be safe to go a year from now, the re-booking option I was offered.

In reality, will Viking be sailing then? And will those with US Passports be allowed to visit any ports?

Is this for the julbord? If so, I doubt there is a dress code. I’d probably wear my standard travel ”dress” outfit which is a pair of black slacks and a blouse.