I am traveling with a group who is staying in Athens for a while and then traveling to Skopelos to stay for around 4 days. We have our place to stay lined up, and know that the ferry schedule is not out yet for December. But any advice for this travel would be great! Any advice on the best port to use, weather, etc.?
Sorry to disappoint a Mamma Mia fan, but Skopelos is in the North Sporades Islands and there are no ferries from Athens.
The ports to reach Skopelos by ferry are either Agios Konstantinos or Volos located more than 150 km from Athens, and in December I'm afraid that you will not have a lot of ferry options.
You can see last year's schedules by going to http://www.openseas.gr/
(enter dates in December 2022)
Another option from Athens would be to fly to Skiathos with Aegean or Sky Express and then find a ferry to Skopelos