December is where peak season starts and the trip turns from landscape to wildlife. Eggs hatch around Christmas, so early December still has adults on nests while late December brings the first chicks. The sun barely sets, more landing sites are open than earlier in the season, and whales are becoming more numerous. It's also the busiest and most expensive part of the year.
Early December is the sweet spot a lot of people miss. You keep the deep snow and pristine sites of the early season, you get sunrises and sunsets before the 24 hour sun arrives, and the pricing is softer than the Christmas peak that starts a couple of weeks later.
Christmas and New Year departures carry a premium driven by holidays and annual leave rather than by anything about the trip being better. If you have flexibility on dates, the same ship on the same route a fortnight either side usually costs meaningfully less.
Temperature runs roughly 32 to 40F by this point, which is about one fleece layer warmer than early season. Not enough to choose a month over.