There Is Going to be a Christmas Special : )

Since "Christmas" as we know it in most English-speaking countries today was introduced by Albert and promoted by Victoria as part of the Gemuetlichkeit family-centric morality she wished to encourage, it would be appropriate.

More than likely PBS will have it be the season 2 finale in March of next year. When "Downton Abbey" had their Christmas specials they just added it to the end of each season. I'm assuming season 2 will air in the fall in the UK but January 2018 here in the US, so the Christmas special will be the season 2 finale in the US airing a week after the last season 2 episode for the UK sometime in March of next year. "Sherlock" so far has been the only UK show PBS has aired the same time day as the UK but that was only these past 2 years as they usually make us wait 3 to 5 months.