If you're on my f-list, you can see the full itinerary. It was somewhat challenging. Now I can explain the business a week ago about "wrong day." We had mistakenly assumed that the Finnlines ferry runs daily. It doesn't. It's six days per week. By bad luck, the original outbound booking put us into Hamburg the same day as we would have had to catch the ferry; otherwise, we would have had to stay an extra day in Hamburg and would have arrived in Helsinki on the first morning of Worldcon, and that would not do on multiple counts. Our plan was to give us one day to recover from the flight (and perhaps see a bit of Hamburg), then arrive in Helsinki early enough that we wouldn't be rattled and rushing to get into place on Day 1. There's no WSFS Business Meeting that day, but I do have WSFS responsibilities there, and I really much prefer to be there at least the night before. As it happens, we now have an extra night in Helsinki pre-con on account of having to travel a day earlier in order to make the ferry crossing work.
A minor bonus: the points basis for the hotel in Helsinki appears to have changed since I originally booked it, from 25K to 20K points/night. So when I modified the 3-night stay to a 4-night stay, IHG processed it by first refunding the original stay (75K) and then deducting the new stay (80K), so the marginal cost of that extra night was only 5K points. Well done, IHG! You keep making me happy that I devote my hotel affiliation with you. Now if only you had a property in Reykjavik....