Steven Moffat has confirmed that the Sherlock Christmas Special will take place in Victorian London, and is a stand-alone episode that doesn't connect with the regular series (see photo above).
Moffat said:
"The special is its own thing; we wouldn’t have done the story we’re doing, and the way we’re doing it, if we didn’t have this special. It’s not part of the run of three episodes. So we had this to do it--as we could hardly conceal--it’s Victorian. Mark Gatiss and me, we wanted to do this, but it had to be a special, it had to be separate entity on its own. It’s kind of in its own little bubble."
No specific airdate has been announced yet, but the special is widely expected to air in the UK this December.