I am thinking of doing a day trip from Helsinki in January 2018 - If we do it on a Sunday, is everything open?
Tallinn is a big, major tourist city. It is not some sleepy, obscure backwater. In the summer, it's downright crowded, mobbed with (mostly) day-trippers off cruise ships. During the busy summer season, you actually need to employ some of the same crowd-dodging tactics one would need to use to enjoy places like Prague or Krakow (ie getting up early to explore the old town free of crowds).
I'm sure that things slow down considerably in the winter and it's not so crowded. But I would not expect everything to close up completely. Certainly there won't be a lot of outdoor cafes and lounging around in the streets (probably too cold and snowy for that in January). But I'm sure most restaurants, shops and other tourist-oriented things in the old town (which is where you want to be) would still be open - albeit with more limited hours. I'd expect it to be more or less similar to Helsinki, so use that as your guide.
It's a charming old city, with oodles of charm. I'm sure it's quite pretty in the snow.
David - I did not say that Tallin was "some sleepy, obscure backwater" place - but I know in some countries that I have traveled
more things are closed on Sundays...and especially in winter, hours may be shortened. doing a day trip so wanted to know how to plan it. Thank you for your response.
You can check on specific attractions via https://www.visittallinn.ee/eng/visitor.